| Message ID | 20200303221423.21962-1-dongli.zhang@oracle.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | cd41648cbfdccfe2ef33cacf02ea0a33b9eb33c2 |
| Headers | show |
| Series | [v3,1/2] xenbus: req->body should be updated before req->state | expand |
Hi, On 03/03/2020 22:14, Dongli Zhang wrote: > The req->body should be updated before req->state is updated and the > order should be guaranteed by a barrier. > > Otherwise, read_reply() might return req->body = NULL. > > Below is sample callstack when the issue is reproduced on purpose by > reordering the updates of req->body and req->state and adding delay in > code between updates of req->state and req->body. > > [ 22.356105] general protection fault: 0000 [#1] SMP PTI > [ 22.361185] CPU: 2 PID: 52 Comm: xenwatch Not tainted 5.5.0xen+ #6 > [ 22.366727] Hardware name: Xen HVM domU, BIOS ... > [ 22.372245] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 > ... ... > [ 22.392163] RSP: 0018:ffffb2d64023fdf0 EFLAGS: 00010246 > [ 22.395933] RAX: 0000000000000000 RBX: 75746e7562755f6d RCX: 0000000000000000 > [ 22.400871] RDX: 0000000000000000 RSI: ffffb2d64023fdfc RDI: 75746e7562755f6d > [ 22.405874] RBP: 0000000000000000 R08: 00000000000001e8 R09: 0000000000cdcdcd > [ 22.410945] R10: ffffb2d6402ffe00 R11: ffff9d95395eaeb0 R12: ffff9d9535935000 > [ 22.417613] R13: ffff9d9526d4a000 R14: ffff9d9526f4f340 R15: ffff9d9537654000 > [ 22.423726] FS: 0000000000000000(0000) GS:ffff9d953bc80000(0000) knlGS:0000000000000000 > [ 22.429898] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [ 22.434342] CR2: 000000c4206a9000 CR3: 00000001ea3fc002 CR4: 00000000001606e0 > [ 22.439645] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > [ 22.444941] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > [ 22.450342] Call Trace: > [ 22.452509] simple_strtoull+0x27/0x70 > [ 22.455572] xenbus_transaction_start+0x31/0x50 > [ 22.459104] netback_changed+0x76c/0xcc1 [xen_netfront] > [ 22.463279] ? find_watch+0x40/0x40 > [ 22.466156] xenwatch_thread+0xb4/0x150 > [ 22.469309] ? wait_woken+0x80/0x80 > [ 22.472198] kthread+0x10e/0x130 > [ 22.474925] ? kthread_park+0x80/0x80 > [ 22.477946] ret_from_fork+0x35/0x40 > [ 22.480968] Modules linked in: xen_kbdfront xen_fbfront(+) xen_netfront xen_blkfront > [ 22.486783] ---[ end trace a9222030a747c3f7 ]--- > [ 22.490424] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 > > The virt_rmb() is added in the 'true' path of test_reply(). The "while" > is changed to "do while" so that test_reply() is used as a read memory > barrier. > > Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com> Reviewed-by: Julien Grall <jgrall@amazon.com> > --- > Changed since v1: > - change "barrier()" to "virt_rmb()" in test_reply() > Changed since v2: > - Use "virt_rmb()" only in 'true' path > > drivers/xen/xenbus/xenbus_comms.c | 2 ++ > drivers/xen/xenbus/xenbus_xs.c | 9 ++++++--- > 2 files changed, 8 insertions(+), 3 deletions(-) > > diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c > index d239fc3c5e3d..852ed161fc2a 100644 > --- a/drivers/xen/xenbus/xenbus_comms.c > +++ b/drivers/xen/xenbus/xenbus_comms.c > @@ -313,6 +313,8 @@ static int process_msg(void) > req->msg.type = state.msg.type; > req->msg.len = state.msg.len; > req->body = state.body; > + /* write body, then update state */ > + virt_wmb(); > req->state = xb_req_state_got_reply; > req->cb(req); > } else > diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c > index ddc18da61834..3a06eb699f33 100644 > --- a/drivers/xen/xenbus/xenbus_xs.c > +++ b/drivers/xen/xenbus/xenbus_xs.c > @@ -191,8 +191,11 @@ static bool xenbus_ok(void) > > static bool test_reply(struct xb_req_data *req) > { > - if (req->state == xb_req_state_got_reply || !xenbus_ok()) > + if (req->state == xb_req_state_got_reply || !xenbus_ok()) { > + /* read req->state before all other fields */ > + virt_rmb(); > return true; > + } > > /* Make sure to reread req->state each time. */ > barrier(); > @@ -202,7 +205,7 @@ static bool test_reply(struct xb_req_data *req) > > static void *read_reply(struct xb_req_data *req) > { > - while (req->state != xb_req_state_got_reply) { > + do { > wait_event(req->wq, test_reply(req)); > > if (!xenbus_ok()) > @@ -216,7 +219,7 @@ static void *read_reply(struct xb_req_data *req) > if (req->err) > return ERR_PTR(req->err); > > - } > + } while (req->state != xb_req_state_got_reply); > > return req->body; > } >
diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c index d239fc3c5e3d..852ed161fc2a 100644 --- a/drivers/xen/xenbus/xenbus_comms.c +++ b/drivers/xen/xenbus/xenbus_comms.c @@ -313,6 +313,8 @@ static int process_msg(void) req->msg.type = state.msg.type; req->msg.len = state.msg.len; req->body = state.body; + /* write body, then update state */ + virt_wmb(); req->state = xb_req_state_got_reply; req->cb(req); } else diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c index ddc18da61834..3a06eb699f33 100644 --- a/drivers/xen/xenbus/xenbus_xs.c +++ b/drivers/xen/xenbus/xenbus_xs.c @@ -191,8 +191,11 @@ static bool xenbus_ok(void) static bool test_reply(struct xb_req_data *req) { - if (req->state == xb_req_state_got_reply || !xenbus_ok()) + if (req->state == xb_req_state_got_reply || !xenbus_ok()) { + /* read req->state before all other fields */ + virt_rmb(); return true; + } /* Make sure to reread req->state each time. */ barrier(); @@ -202,7 +205,7 @@ static bool test_reply(struct xb_req_data *req) static void *read_reply(struct xb_req_data *req) { - while (req->state != xb_req_state_got_reply) { + do { wait_event(req->wq, test_reply(req)); if (!xenbus_ok()) @@ -216,7 +219,7 @@ static void *read_reply(struct xb_req_data *req) if (req->err) return ERR_PTR(req->err); - } + } while (req->state != xb_req_state_got_reply); return req->body; }
The req->body should be updated before req->state is updated and the order should be guaranteed by a barrier. Otherwise, read_reply() might return req->body = NULL. Below is sample callstack when the issue is reproduced on purpose by reordering the updates of req->body and req->state and adding delay in code between updates of req->state and req->body. [ 22.356105] general protection fault: 0000 [#1] SMP PTI [ 22.361185] CPU: 2 PID: 52 Comm: xenwatch Not tainted 5.5.0xen+ #6 [ 22.366727] Hardware name: Xen HVM domU, BIOS ... [ 22.372245] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 ... ... [ 22.392163] RSP: 0018:ffffb2d64023fdf0 EFLAGS: 00010246 [ 22.395933] RAX: 0000000000000000 RBX: 75746e7562755f6d RCX: 0000000000000000 [ 22.400871] RDX: 0000000000000000 RSI: ffffb2d64023fdfc RDI: 75746e7562755f6d [ 22.405874] RBP: 0000000000000000 R08: 00000000000001e8 R09: 0000000000cdcdcd [ 22.410945] R10: ffffb2d6402ffe00 R11: ffff9d95395eaeb0 R12: ffff9d9535935000 [ 22.417613] R13: ffff9d9526d4a000 R14: ffff9d9526f4f340 R15: ffff9d9537654000 [ 22.423726] FS: 0000000000000000(0000) GS:ffff9d953bc80000(0000) knlGS:0000000000000000 [ 22.429898] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 22.434342] CR2: 000000c4206a9000 CR3: 00000001ea3fc002 CR4: 00000000001606e0 [ 22.439645] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 22.444941] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 22.450342] Call Trace: [ 22.452509] simple_strtoull+0x27/0x70 [ 22.455572] xenbus_transaction_start+0x31/0x50 [ 22.459104] netback_changed+0x76c/0xcc1 [xen_netfront] [ 22.463279] ? find_watch+0x40/0x40 [ 22.466156] xenwatch_thread+0xb4/0x150 [ 22.469309] ? wait_woken+0x80/0x80 [ 22.472198] kthread+0x10e/0x130 [ 22.474925] ? kthread_park+0x80/0x80 [ 22.477946] ret_from_fork+0x35/0x40 [ 22.480968] Modules linked in: xen_kbdfront xen_fbfront(+) xen_netfront xen_blkfront [ 22.486783] ---[ end trace a9222030a747c3f7 ]--- [ 22.490424] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 The virt_rmb() is added in the 'true' path of test_reply(). The "while" is changed to "do while" so that test_reply() is used as a read memory barrier. Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com> --- Changed since v1: - change "barrier()" to "virt_rmb()" in test_reply() Changed since v2: - Use "virt_rmb()" only in 'true' path drivers/xen/xenbus/xenbus_comms.c | 2 ++ drivers/xen/xenbus/xenbus_xs.c | 9 ++++++--- 2 files changed, 8 insertions(+), 3 deletions(-)