| Message ID | 20210818073824.1560124-8-desmondcheongzx@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | drm, kernel: update locking for DRM | expand |
On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote: > In a future patch, a read lock on drm_device.master_rwsem is > held in the ioctl handler before the check for ioctl > permissions. However, this produces the following lockdep splat: > > ====================================================== > WARNING: possible circular locking dependency detected > 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U > ------------------------------------------------------ > kms_lease/1752 is trying to acquire lock: > ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280 > > but task is already holding lock: > ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: > drm_ioctl_kernel+0xfb/0x1a0 > > which lock already depends on the new lock. > > the existing dependency chain (in reverse order) is: > > -> #2 (&dev->master_rwsem){++++}-{3:3}: > lock_acquire+0xd3/0x310 > down_read+0x3b/0x140 > drm_master_internal_acquire+0x1d/0x60 > drm_client_modeset_commit+0x10/0x40 > __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 > drm_fb_helper_set_par+0x34/0x40 > intel_fbdev_set_par+0x11/0x40 [i915] > fbcon_init+0x270/0x4f0 > visual_init+0xc6/0x130 > do_bind_con_driver+0x1de/0x2c0 > do_take_over_console+0x10e/0x180 > do_fbcon_takeover+0x53/0xb0 > register_framebuffer+0x22d/0x310 > __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 > intel_fbdev_initial_config+0xf/0x20 [i915] > async_run_entry_fn+0x28/0x130 > process_one_work+0x26d/0x5c0 > worker_thread+0x37/0x390 > kthread+0x13b/0x170 > ret_from_fork+0x1f/0x30 > > -> #1 (&helper->lock){+.+.}-{3:3}: > lock_acquire+0xd3/0x310 > __mutex_lock+0xa8/0x930 > __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 > intel_fbdev_restore_mode+0x2b/0x50 [i915] > drm_lastclose+0x27/0x50 > drm_release_noglobal+0x42/0x60 > __fput+0x9e/0x250 > task_work_run+0x6b/0xb0 > exit_to_user_mode_prepare+0x1c5/0x1d0 > syscall_exit_to_user_mode+0x19/0x50 > do_syscall_64+0x46/0xb0 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > -> #0 (drm_global_mutex){+.+.}-{3:3}: > validate_chain+0xb39/0x1e70 > __lock_acquire+0x5a1/0xb70 > lock_acquire+0xd3/0x310 > __mutex_lock+0xa8/0x930 > drm_open+0x64/0x280 > drm_stub_open+0x9f/0x100 > chrdev_open+0x9f/0x1d0 > do_dentry_open+0x14a/0x3a0 > dentry_open+0x53/0x70 > drm_mode_create_lease_ioctl+0x3cb/0x970 > drm_ioctl_kernel+0xc9/0x1a0 > drm_ioctl+0x201/0x3d0 > __x64_sys_ioctl+0x6a/0xa0 > do_syscall_64+0x37/0xb0 > entry_SYSCALL_64_after_hwframe+0x44/0xae > > other info that might help us debug this: > Chain exists of: > drm_global_mutex --> &helper->lock --> &dev->master_rwsem > Possible unsafe locking scenario: > CPU0 CPU1 > ---- ---- > lock(&dev->master_rwsem); > lock(&helper->lock); > lock(&dev->master_rwsem); > lock(drm_global_mutex); > > *** DEADLOCK *** > > The lock hierarchy inversion happens because we grab the > drm_global_mutex while already holding on to master_rwsem. To avoid > this, we do some prep work to grab the drm_global_mutex before > checking for ioctl permissions. > > At the same time, we update the check for the global mutex to use the > drm_dev_needs_global_mutex helper function. This is intentional, essentially we force all non-legacy drivers to have unlocked ioctl (otherwise everyone forgets to set that flag). For non-legacy drivers the global lock only ensures ordering between drm_open and lastclose (I think at least), and between drm_dev_register/unregister and the backwards ->load/unload callbacks (which are called in the wrong place, but we cannot fix that for legacy drivers). ->load/unload should be completely unused (maybe radeon still uses it), and ->lastclose is also on the decline. Maybe we should update the comment of drm_global_mutex to explain what it protects and why. I'm also confused how this patch connects to the splat, since for i915 we shouldn't be taking the drm_global_lock here at all. The problem seems to be the drm_open_helper when we create a new lease, which is an entirely different can of worms. I'm honestly not sure how to best do that, but we should be able to create a file and then call drm_open_helper directly, or well a version of that which never takes the drm_global_mutex. Because that is not needed for nested drm_file opening: - legacy drivers never go down this path because leases are only supported with modesetting, and modesetting is only supported for non-legacy drivers - the races against dev->open_count due to last_close or ->load callbacks don't matter, because for the entire ioctl we already have an open drm_file and that wont disappear. So this should work, but I'm not entirely sure how to make it work. -Daniel > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> > --- > drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- > 1 file changed, 9 insertions(+), 9 deletions(-) > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > index 880fc565d599..2cb57378a787 100644 > --- a/drivers/gpu/drm/drm_ioctl.c > +++ b/drivers/gpu/drm/drm_ioctl.c > @@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, > if (drm_dev_is_unplugged(dev)) > return -ENODEV; > > + /* Enforce sane locking for modern driver ioctls. */ > + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) > + mutex_lock(&drm_global_mutex); > + > retcode = drm_ioctl_permit(flags, file_priv); > if (unlikely(retcode)) > - return retcode; > + goto out; > > - /* Enforce sane locking for modern driver ioctls. */ > - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || > - (flags & DRM_UNLOCKED)) > - retcode = func(dev, kdata, file_priv); > - else { > - mutex_lock(&drm_global_mutex); > - retcode = func(dev, kdata, file_priv); > + retcode = func(dev, kdata, file_priv); > + > +out: > + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) > mutex_unlock(&drm_global_mutex); > - } > return retcode; > } > EXPORT_SYMBOL(drm_ioctl_kernel); > -- > 2.25.1 >
On 18/8/21 7:02 pm, Daniel Vetter wrote: > On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote: >> In a future patch, a read lock on drm_device.master_rwsem is >> held in the ioctl handler before the check for ioctl >> permissions. However, this produces the following lockdep splat: >> >> ====================================================== >> WARNING: possible circular locking dependency detected >> 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U >> ------------------------------------------------------ >> kms_lease/1752 is trying to acquire lock: >> ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280 >> >> but task is already holding lock: >> ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: >> drm_ioctl_kernel+0xfb/0x1a0 >> >> which lock already depends on the new lock. >> >> the existing dependency chain (in reverse order) is: >> >> -> #2 (&dev->master_rwsem){++++}-{3:3}: >> lock_acquire+0xd3/0x310 >> down_read+0x3b/0x140 >> drm_master_internal_acquire+0x1d/0x60 >> drm_client_modeset_commit+0x10/0x40 >> __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 >> drm_fb_helper_set_par+0x34/0x40 >> intel_fbdev_set_par+0x11/0x40 [i915] >> fbcon_init+0x270/0x4f0 >> visual_init+0xc6/0x130 >> do_bind_con_driver+0x1de/0x2c0 >> do_take_over_console+0x10e/0x180 >> do_fbcon_takeover+0x53/0xb0 >> register_framebuffer+0x22d/0x310 >> __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 >> intel_fbdev_initial_config+0xf/0x20 [i915] >> async_run_entry_fn+0x28/0x130 >> process_one_work+0x26d/0x5c0 >> worker_thread+0x37/0x390 >> kthread+0x13b/0x170 >> ret_from_fork+0x1f/0x30 >> >> -> #1 (&helper->lock){+.+.}-{3:3}: >> lock_acquire+0xd3/0x310 >> __mutex_lock+0xa8/0x930 >> __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 >> intel_fbdev_restore_mode+0x2b/0x50 [i915] >> drm_lastclose+0x27/0x50 >> drm_release_noglobal+0x42/0x60 >> __fput+0x9e/0x250 >> task_work_run+0x6b/0xb0 >> exit_to_user_mode_prepare+0x1c5/0x1d0 >> syscall_exit_to_user_mode+0x19/0x50 >> do_syscall_64+0x46/0xb0 >> entry_SYSCALL_64_after_hwframe+0x44/0xae >> >> -> #0 (drm_global_mutex){+.+.}-{3:3}: >> validate_chain+0xb39/0x1e70 >> __lock_acquire+0x5a1/0xb70 >> lock_acquire+0xd3/0x310 >> __mutex_lock+0xa8/0x930 >> drm_open+0x64/0x280 >> drm_stub_open+0x9f/0x100 >> chrdev_open+0x9f/0x1d0 >> do_dentry_open+0x14a/0x3a0 >> dentry_open+0x53/0x70 >> drm_mode_create_lease_ioctl+0x3cb/0x970 >> drm_ioctl_kernel+0xc9/0x1a0 >> drm_ioctl+0x201/0x3d0 >> __x64_sys_ioctl+0x6a/0xa0 >> do_syscall_64+0x37/0xb0 >> entry_SYSCALL_64_after_hwframe+0x44/0xae >> >> other info that might help us debug this: >> Chain exists of: >> drm_global_mutex --> &helper->lock --> &dev->master_rwsem >> Possible unsafe locking scenario: >> CPU0 CPU1 >> ---- ---- >> lock(&dev->master_rwsem); >> lock(&helper->lock); >> lock(&dev->master_rwsem); >> lock(drm_global_mutex); >> >> *** DEADLOCK *** >> >> The lock hierarchy inversion happens because we grab the >> drm_global_mutex while already holding on to master_rwsem. To avoid >> this, we do some prep work to grab the drm_global_mutex before >> checking for ioctl permissions. >> >> At the same time, we update the check for the global mutex to use the >> drm_dev_needs_global_mutex helper function. > > This is intentional, essentially we force all non-legacy drivers to have > unlocked ioctl (otherwise everyone forgets to set that flag). > > For non-legacy drivers the global lock only ensures ordering between > drm_open and lastclose (I think at least), and between > drm_dev_register/unregister and the backwards ->load/unload callbacks > (which are called in the wrong place, but we cannot fix that for legacy > drivers). > > ->load/unload should be completely unused (maybe radeon still uses it), > and ->lastclose is also on the decline. > Ah ok got it, I'll change the check back to drm_core_check_feature(dev, DRIVER_LEGACY) then. > Maybe we should update the comment of drm_global_mutex to explain what it > protects and why. > The comments in drm_dev_needs_global_mutex make sense I think, I just didn't read the code closely enough. > I'm also confused how this patch connects to the splat, since for i915 we Right, my bad, this is a separate instance of circular locking. I was too hasty when I saw that for legacy drivers we might grab master_rwsem then drm_global_mutex in the ioctl handler. > shouldn't be taking the drm_global_lock here at all. The problem seems to > be the drm_open_helper when we create a new lease, which is an entirely > different can of worms. > > I'm honestly not sure how to best do that, but we should be able to create > a file and then call drm_open_helper directly, or well a version of that > which never takes the drm_global_mutex. Because that is not needed for > nested drm_file opening: > - legacy drivers never go down this path because leases are only supported > with modesetting, and modesetting is only supported for non-legacy > drivers > - the races against dev->open_count due to last_close or ->load callbacks > don't matter, because for the entire ioctl we already have an open > drm_file and that wont disappear. > > So this should work, but I'm not entirely sure how to make it work. > -Daniel > One idea that comes to mind is to change the outcome of drm_dev_needs_global_mutex while we're in the ioctl, but that requires more locking which sounds like a bad idea. Another idea, which is quite messy, but just for thoughts, uses the idea of pushing the master_rwsem read lock down: diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 7f523e1c5650..5d05e744b728 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -712,7 +712,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = { DRM_RENDER_ALLOW), DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0), DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0), - DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER), + DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0), DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER), DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER), DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER), diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c index 983701198ffd..a25bc69522b4 100644 --- a/drivers/gpu/drm/drm_lease.c +++ b/drivers/gpu/drm/drm_lease.c @@ -500,6 +500,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, return -EINVAL; } + /* Clone the lessor file to create a new file for us */ + DRM_DEBUG_LEASE("Allocating lease file\n"); + lessee_file = file_clone_open(lessor_file); + if (IS_ERR(lessee_file)) + return PTR_ERR(lessee_file); + + down_read(&dev->master_rwsem); + + if (!drm_is_current_master(lessor_priv)) { + ret = -EACCES; + goto out_file; + } + lessor = drm_file_get_master(lessor_priv); /* Do not allow sub-leases */ if (lessor->lessor) { @@ -547,14 +560,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, goto out_leases; } - /* Clone the lessor file to create a new file for us */ - DRM_DEBUG_LEASE("Allocating lease file\n"); - lessee_file = file_clone_open(lessor_file); - if (IS_ERR(lessee_file)) { - ret = PTR_ERR(lessee_file); - goto out_lessee; - } - lessee_priv = lessee_file->private_data; /* Change the file to a master one */ drm_master_put(&lessee_priv->master); @@ -571,17 +576,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, fd_install(fd, lessee_file); drm_master_put(&lessor); + up_read(&dev->master_rwsem); DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n"); return 0; -out_lessee: - drm_master_put(&lessee); - out_leases: put_unused_fd(fd); out_lessor: drm_master_put(&lessor); + +out_file: + up_read(&dev->master_rwsem); + fput(lessee_file); DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret); return ret; } Something like this would also address the other deadlock we'd hit in drm_mode_create_lease_ioctl(): drm_ioctl_kernel(): down_read(&master_rwsem); <--- down_read() drm_mode_create_lease_ioctl(): drm_lease_create(): file_clone_open(): ... drm_open(): drm_open_helper(): drm_master_open(): down_write(&master_rwsem); <--- down_write() Overall, I think the suggestion to push master_rwsem write locks down into ioctls would solve the nesting problem for those ioctls. Although I'm still a little concerned that, just like here, there might be deeply embedded nested locking, so locking becomes prone to breaking. It does smell a bit to me. >> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> >> --- >> drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- >> 1 file changed, 9 insertions(+), 9 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c >> index 880fc565d599..2cb57378a787 100644 >> --- a/drivers/gpu/drm/drm_ioctl.c >> +++ b/drivers/gpu/drm/drm_ioctl.c >> @@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, >> if (drm_dev_is_unplugged(dev)) >> return -ENODEV; >> >> + /* Enforce sane locking for modern driver ioctls. */ >> + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) >> + mutex_lock(&drm_global_mutex); >> + >> retcode = drm_ioctl_permit(flags, file_priv); >> if (unlikely(retcode)) >> - return retcode; >> + goto out; >> >> - /* Enforce sane locking for modern driver ioctls. */ >> - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || >> - (flags & DRM_UNLOCKED)) >> - retcode = func(dev, kdata, file_priv); >> - else { >> - mutex_lock(&drm_global_mutex); >> - retcode = func(dev, kdata, file_priv); >> + retcode = func(dev, kdata, file_priv); >> + >> +out: >> + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) >> mutex_unlock(&drm_global_mutex); >> - } >> return retcode; >> } >> EXPORT_SYMBOL(drm_ioctl_kernel); >> -- >> 2.25.1 >> >
On Thu, Aug 19, 2021 at 12:53 PM Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> wrote: > > On 18/8/21 7:02 pm, Daniel Vetter wrote: > > On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote: > >> In a future patch, a read lock on drm_device.master_rwsem is > >> held in the ioctl handler before the check for ioctl > >> permissions. However, this produces the following lockdep splat: > >> > >> ====================================================== > >> WARNING: possible circular locking dependency detected > >> 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U > >> ------------------------------------------------------ > >> kms_lease/1752 is trying to acquire lock: > >> ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280 > >> > >> but task is already holding lock: > >> ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: > >> drm_ioctl_kernel+0xfb/0x1a0 > >> > >> which lock already depends on the new lock. > >> > >> the existing dependency chain (in reverse order) is: > >> > >> -> #2 (&dev->master_rwsem){++++}-{3:3}: > >> lock_acquire+0xd3/0x310 > >> down_read+0x3b/0x140 > >> drm_master_internal_acquire+0x1d/0x60 > >> drm_client_modeset_commit+0x10/0x40 > >> __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 > >> drm_fb_helper_set_par+0x34/0x40 > >> intel_fbdev_set_par+0x11/0x40 [i915] > >> fbcon_init+0x270/0x4f0 > >> visual_init+0xc6/0x130 > >> do_bind_con_driver+0x1de/0x2c0 > >> do_take_over_console+0x10e/0x180 > >> do_fbcon_takeover+0x53/0xb0 > >> register_framebuffer+0x22d/0x310 > >> __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 > >> intel_fbdev_initial_config+0xf/0x20 [i915] > >> async_run_entry_fn+0x28/0x130 > >> process_one_work+0x26d/0x5c0 > >> worker_thread+0x37/0x390 > >> kthread+0x13b/0x170 > >> ret_from_fork+0x1f/0x30 > >> > >> -> #1 (&helper->lock){+.+.}-{3:3}: > >> lock_acquire+0xd3/0x310 > >> __mutex_lock+0xa8/0x930 > >> __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 > >> intel_fbdev_restore_mode+0x2b/0x50 [i915] > >> drm_lastclose+0x27/0x50 > >> drm_release_noglobal+0x42/0x60 > >> __fput+0x9e/0x250 > >> task_work_run+0x6b/0xb0 > >> exit_to_user_mode_prepare+0x1c5/0x1d0 > >> syscall_exit_to_user_mode+0x19/0x50 > >> do_syscall_64+0x46/0xb0 > >> entry_SYSCALL_64_after_hwframe+0x44/0xae > >> > >> -> #0 (drm_global_mutex){+.+.}-{3:3}: > >> validate_chain+0xb39/0x1e70 > >> __lock_acquire+0x5a1/0xb70 > >> lock_acquire+0xd3/0x310 > >> __mutex_lock+0xa8/0x930 > >> drm_open+0x64/0x280 > >> drm_stub_open+0x9f/0x100 > >> chrdev_open+0x9f/0x1d0 > >> do_dentry_open+0x14a/0x3a0 > >> dentry_open+0x53/0x70 > >> drm_mode_create_lease_ioctl+0x3cb/0x970 > >> drm_ioctl_kernel+0xc9/0x1a0 > >> drm_ioctl+0x201/0x3d0 > >> __x64_sys_ioctl+0x6a/0xa0 > >> do_syscall_64+0x37/0xb0 > >> entry_SYSCALL_64_after_hwframe+0x44/0xae > >> > >> other info that might help us debug this: > >> Chain exists of: > >> drm_global_mutex --> &helper->lock --> &dev->master_rwsem > >> Possible unsafe locking scenario: > >> CPU0 CPU1 > >> ---- ---- > >> lock(&dev->master_rwsem); > >> lock(&helper->lock); > >> lock(&dev->master_rwsem); > >> lock(drm_global_mutex); > >> > >> *** DEADLOCK *** > >> > >> The lock hierarchy inversion happens because we grab the > >> drm_global_mutex while already holding on to master_rwsem. To avoid > >> this, we do some prep work to grab the drm_global_mutex before > >> checking for ioctl permissions. > >> > >> At the same time, we update the check for the global mutex to use the > >> drm_dev_needs_global_mutex helper function. > > > > This is intentional, essentially we force all non-legacy drivers to have > > unlocked ioctl (otherwise everyone forgets to set that flag). > > > > For non-legacy drivers the global lock only ensures ordering between > > drm_open and lastclose (I think at least), and between > > drm_dev_register/unregister and the backwards ->load/unload callbacks > > (which are called in the wrong place, but we cannot fix that for legacy > > drivers). > > > > ->load/unload should be completely unused (maybe radeon still uses it), > > and ->lastclose is also on the decline. > > > > Ah ok got it, I'll change the check back to > drm_core_check_feature(dev, DRIVER_LEGACY) then. > > > Maybe we should update the comment of drm_global_mutex to explain what it > > protects and why. > > > > The comments in drm_dev_needs_global_mutex make sense I think, I just > didn't read the code closely enough. > > > I'm also confused how this patch connects to the splat, since for i915 we > > Right, my bad, this is a separate instance of circular locking. I was > too hasty when I saw that for legacy drivers we might grab master_rwsem > then drm_global_mutex in the ioctl handler. > > > shouldn't be taking the drm_global_lock here at all. The problem seems to > > be the drm_open_helper when we create a new lease, which is an entirely > > different can of worms. > > > > I'm honestly not sure how to best do that, but we should be able to create > > a file and then call drm_open_helper directly, or well a version of that > > which never takes the drm_global_mutex. Because that is not needed for > > nested drm_file opening: > > - legacy drivers never go down this path because leases are only supported > > with modesetting, and modesetting is only supported for non-legacy > > drivers > > - the races against dev->open_count due to last_close or ->load callbacks > > don't matter, because for the entire ioctl we already have an open > > drm_file and that wont disappear. > > > > So this should work, but I'm not entirely sure how to make it work. > > -Daniel > > > > One idea that comes to mind is to change the outcome of > drm_dev_needs_global_mutex while we're in the ioctl, but that requires > more locking which sounds like a bad idea. > > Another idea, which is quite messy, but just for thoughts, uses the idea > of pushing the master_rwsem read lock down: Yeah I think that's cleaner, and I think that also should work a lot better for the other ioctls: - We don't have a need to flush readers anymore since we'll just take the rwsem in write mode - There's much less inversions, and maybe we could even get rid of the spinlock since at that point all readers should at least have the rwsem read-locked. > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > index 7f523e1c5650..5d05e744b728 100644 > --- a/drivers/gpu/drm/drm_ioctl.c > +++ b/drivers/gpu/drm/drm_ioctl.c > @@ -712,7 +712,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = { > DRM_RENDER_ALLOW), > DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0), > DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0), > - DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER), > + DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0), > DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER), > DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER), > DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER), > diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c > index 983701198ffd..a25bc69522b4 100644 > --- a/drivers/gpu/drm/drm_lease.c > +++ b/drivers/gpu/drm/drm_lease.c > @@ -500,6 +500,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, > return -EINVAL; > } > > + /* Clone the lessor file to create a new file for us */ > + DRM_DEBUG_LEASE("Allocating lease file\n"); > + lessee_file = file_clone_open(lessor_file); > + if (IS_ERR(lessee_file)) > + return PTR_ERR(lessee_file); > + > + down_read(&dev->master_rwsem); > + > + if (!drm_is_current_master(lessor_priv)) { > + ret = -EACCES; > + goto out_file; > + } > + > lessor = drm_file_get_master(lessor_priv); > /* Do not allow sub-leases */ > if (lessor->lessor) { > @@ -547,14 +560,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, > goto out_leases; > } > > - /* Clone the lessor file to create a new file for us */ > - DRM_DEBUG_LEASE("Allocating lease file\n"); > - lessee_file = file_clone_open(lessor_file); > - if (IS_ERR(lessee_file)) { > - ret = PTR_ERR(lessee_file); > - goto out_lessee; > - } > - > lessee_priv = lessee_file->private_data; > /* Change the file to a master one */ > drm_master_put(&lessee_priv->master); > @@ -571,17 +576,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev, > fd_install(fd, lessee_file); > > drm_master_put(&lessor); > + up_read(&dev->master_rwsem); > DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n"); > return 0; > > -out_lessee: > - drm_master_put(&lessee); > - > out_leases: > put_unused_fd(fd); > > out_lessor: > drm_master_put(&lessor); > + > +out_file: > + up_read(&dev->master_rwsem); > + fput(lessee_file); > DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret); > return ret; > } > > > Something like this would also address the other deadlock we'd hit in > drm_mode_create_lease_ioctl(): > > drm_ioctl_kernel(): > down_read(&master_rwsem); <--- down_read() > drm_mode_create_lease_ioctl(): > drm_lease_create(): > file_clone_open(): > ... > drm_open(): > drm_open_helper(): > drm_master_open(): > down_write(&master_rwsem); <--- down_write() > > Overall, I think the suggestion to push master_rwsem write locks down > into ioctls would solve the nesting problem for those ioctls. Yup, my gut feeling agress. And the above is a nice solution without having to dig out all the code for creating a file directly (it's doable I think at least, we do it for dma-buf). > Although I'm still a little concerned that, just like here, there might > be deeply embedded nested locking, so locking becomes prone to breaking. > It does smell a bit to me. Yeah, that's pretty much the bane of locking cleanup/rework. You have to do it to figure out what goes boom :-/ Even with the most careful audit there's surprises left. -Daniel > >> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> > >> --- > >> drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- > >> 1 file changed, 9 insertions(+), 9 deletions(-) > >> > >> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > >> index 880fc565d599..2cb57378a787 100644 > >> --- a/drivers/gpu/drm/drm_ioctl.c > >> +++ b/drivers/gpu/drm/drm_ioctl.c > >> @@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, > >> if (drm_dev_is_unplugged(dev)) > >> return -ENODEV; > >> > >> + /* Enforce sane locking for modern driver ioctls. */ > >> + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) > >> + mutex_lock(&drm_global_mutex); > >> + > >> retcode = drm_ioctl_permit(flags, file_priv); > >> if (unlikely(retcode)) > >> - return retcode; > >> + goto out; > >> > >> - /* Enforce sane locking for modern driver ioctls. */ > >> - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || > >> - (flags & DRM_UNLOCKED)) > >> - retcode = func(dev, kdata, file_priv); > >> - else { > >> - mutex_lock(&drm_global_mutex); > >> - retcode = func(dev, kdata, file_priv); > >> + retcode = func(dev, kdata, file_priv); > >> + > >> +out: > >> + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) > >> mutex_unlock(&drm_global_mutex); > >> - } > >> return retcode; > >> } > >> EXPORT_SYMBOL(drm_ioctl_kernel); > >> -- > >> 2.25.1 > >> > > >
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 880fc565d599..2cb57378a787 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, if (drm_dev_is_unplugged(dev)) return -ENODEV; + /* Enforce sane locking for modern driver ioctls. */ + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) + mutex_lock(&drm_global_mutex); + retcode = drm_ioctl_permit(flags, file_priv); if (unlikely(retcode)) - return retcode; + goto out; - /* Enforce sane locking for modern driver ioctls. */ - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) || - (flags & DRM_UNLOCKED)) - retcode = func(dev, kdata, file_priv); - else { - mutex_lock(&drm_global_mutex); - retcode = func(dev, kdata, file_priv); + retcode = func(dev, kdata, file_priv); + +out: + if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) mutex_unlock(&drm_global_mutex); - } return retcode; } EXPORT_SYMBOL(drm_ioctl_kernel);
In a future patch, a read lock on drm_device.master_rwsem is held in the ioctl handler before the check for ioctl permissions. However, this produces the following lockdep splat: ====================================================== WARNING: possible circular locking dependency detected 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U ------------------------------------------------------ kms_lease/1752 is trying to acquire lock: ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280 but task is already holding lock: ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: drm_ioctl_kernel+0xfb/0x1a0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (&dev->master_rwsem){++++}-{3:3}: lock_acquire+0xd3/0x310 down_read+0x3b/0x140 drm_master_internal_acquire+0x1d/0x60 drm_client_modeset_commit+0x10/0x40 __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 drm_fb_helper_set_par+0x34/0x40 intel_fbdev_set_par+0x11/0x40 [i915] fbcon_init+0x270/0x4f0 visual_init+0xc6/0x130 do_bind_con_driver+0x1de/0x2c0 do_take_over_console+0x10e/0x180 do_fbcon_takeover+0x53/0xb0 register_framebuffer+0x22d/0x310 __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 intel_fbdev_initial_config+0xf/0x20 [i915] async_run_entry_fn+0x28/0x130 process_one_work+0x26d/0x5c0 worker_thread+0x37/0x390 kthread+0x13b/0x170 ret_from_fork+0x1f/0x30 -> #1 (&helper->lock){+.+.}-{3:3}: lock_acquire+0xd3/0x310 __mutex_lock+0xa8/0x930 __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 intel_fbdev_restore_mode+0x2b/0x50 [i915] drm_lastclose+0x27/0x50 drm_release_noglobal+0x42/0x60 __fput+0x9e/0x250 task_work_run+0x6b/0xb0 exit_to_user_mode_prepare+0x1c5/0x1d0 syscall_exit_to_user_mode+0x19/0x50 do_syscall_64+0x46/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae -> #0 (drm_global_mutex){+.+.}-{3:3}: validate_chain+0xb39/0x1e70 __lock_acquire+0x5a1/0xb70 lock_acquire+0xd3/0x310 __mutex_lock+0xa8/0x930 drm_open+0x64/0x280 drm_stub_open+0x9f/0x100 chrdev_open+0x9f/0x1d0 do_dentry_open+0x14a/0x3a0 dentry_open+0x53/0x70 drm_mode_create_lease_ioctl+0x3cb/0x970 drm_ioctl_kernel+0xc9/0x1a0 drm_ioctl+0x201/0x3d0 __x64_sys_ioctl+0x6a/0xa0 do_syscall_64+0x37/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae other info that might help us debug this: Chain exists of: drm_global_mutex --> &helper->lock --> &dev->master_rwsem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&dev->master_rwsem); lock(&helper->lock); lock(&dev->master_rwsem); lock(drm_global_mutex); *** DEADLOCK *** The lock hierarchy inversion happens because we grab the drm_global_mutex while already holding on to master_rwsem. To avoid this, we do some prep work to grab the drm_global_mutex before checking for ioctl permissions. At the same time, we update the check for the global mutex to use the drm_dev_needs_global_mutex helper function. Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> --- drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-)