| Message ID | 20220927014823.11439-1-zhouzhouyi@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [linux-next,RFC] powerpc: avoid lockdep when we are offline | expand |
On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > This is second version of my fix to PPC's "WARNING: suspicious RCU usage", > I improved my fix under Paul E. McKenney's guidance: > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ > > During the cpu offlining, the sub functions of xive_teardown_cpu will > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will > travel RCU protected list, so "WARNING: suspicious RCU usage" will be > triggered. > > Avoid lockdep when we are offline. I don't see how this is safe. If RCU is no longer watching the CPU then the memory it is accessing here could be concurrently freed. I think the warning is valid. powerpc's problem is that cpuhp_report_idle_dead() is called before arch_cpu_idle_dead(), so it must not rely on any RCU protection there. I would say xive cleanup just needs to be done earlier. I wonder why it is not done in __cpu_disable or thereabouts, that's where the interrupt controller is supposed to be stopped. Thanks, Nick > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > --- > Dear PPC and RCU developers > > I found this bug when trying to do rcutorture tests in ppc VM of > Open Source Lab of Oregon State University > > console.log report following bug: > [ 37.635545][ T0] WARNING: suspicious RCU usage^M > [ 37.636409][ T0] 6.0.0-rc4-next-20220907-dirty #8 Not tainted^M > [ 37.637575][ T0] -----------------------------^M > [ 37.638306][ T0] kernel/locking/lockdep.c:3723 RCU-list traversed in non-reader section!!^M > [ 37.639651][ T0] ^M > [ 37.639651][ T0] other info that might help us debug this:^M > [ 37.639651][ T0] ^M > [ 37.641381][ T0] ^M > [ 37.641381][ T0] RCU used illegally from offline CPU!^M > [ 37.641381][ T0] rcu_scheduler_active = 2, debug_locks = 1^M > [ 37.667170][ T0] no locks held by swapper/6/0.^M > [ 37.668328][ T0] ^M > [ 37.668328][ T0] stack backtrace:^M > [ 37.669995][ T0] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 6.0.0-rc4-next-20220907-dirty #8^M > [ 37.672777][ T0] Call Trace:^M > [ 37.673729][ T0] [c000000004653920] [c00000000097f9b4] dump_stack_lvl+0x98/0xe0 (unreliable)^M > [ 37.678579][ T0] [c000000004653960] [c0000000001f2eb8] lockdep_rcu_suspicious+0x148/0x16c^M > [ 37.680425][ T0] [c0000000046539f0] [c0000000001ed9b4] __lock_acquire+0x10f4/0x26e0^M > [ 37.682450][ T0] [c000000004653b30] [c0000000001efc2c] lock_acquire+0x12c/0x420^M > [ 37.684113][ T0] [c000000004653c20] [c0000000010d704c] _raw_spin_lock_irqsave+0x6c/0xc0^M > [ 37.686154][ T0] [c000000004653c60] [c0000000000c7b4c] xive_spapr_put_ipi+0xcc/0x150^M > [ 37.687879][ T0] [c000000004653ca0] [c0000000010c72a8] xive_cleanup_cpu_ipi+0xc8/0xf0^M > [ 37.689856][ T0] [c000000004653cf0] [c0000000010c7370] xive_teardown_cpu+0xa0/0xf0^M > [ 37.691877][ T0] [c000000004653d30] [c0000000000fba5c] pseries_cpu_offline_self+0x5c/0x100^M > [ 37.693882][ T0] [c000000004653da0] [c00000000005d2c4] arch_cpu_idle_dead+0x44/0x60^M > [ 37.695739][ T0] [c000000004653dc0] [c0000000001c740c] do_idle+0x16c/0x3d0^M > [ 37.697536][ T0] [c000000004653e70] [c0000000001c7a1c] cpu_startup_entry+0x3c/0x40^M > [ 37.699694][ T0] [c000000004653ea0] [c00000000005ca20] start_secondary+0x6c0/0xb50^M > [ 37.701742][ T0] [c000000004653f90] [c00000000000d054] start_secondary_prolog+0x10/0x14^M > > > Tested on PPC VM of Open Source Lab of Oregon State University. > Test results show that although "WARNING: suspicious RCU usage" has gone, > and there are less "BUG: soft lockup" reports than the original kernel > (9 vs 13), which sounds good ;-) > > But after my modification, results-rcutorture-kasan/SRCU-P/console.log.diags > shows a new warning: > [ 222.289242][ T110] WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0 > > I guess above new warning also exits in original kernel, so I write a tiny test script as follows: > > #!/bin/sh > > COUNTER=0 > while [ $COUNTER -lt 1000 ] ; do > qemu-system-ppc64 -nographic -smp cores=8,threads=1 -net none -M pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log -m 2G -kernel /tmp/vmlinux -append "debug_boot_weak_hash panic=-1 console=ttyS0 rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 rcutorture.torture_type=srcud rcupdate.rcu_self_test=1 rcutorture.fwd_progress=3 srcutree.big_cpu_lim=5 rcutorture.onoff_interval=1000 rcutorture.onoff_holdoff=30 rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15 rcutorture.shutdown_secs=420 rcutorture.test_no_idle_hz=1 rcutorture.verbose=1"& > qemu_pid=$! > cd ~/next1/linux-next > make clean > #I use "make vmlinux -j 8" to create heavy background jitter > make vmlinux -j 8 > /dev/null 2>&1 > make_pid=$! > wait $qemu_pid > kill $qemu_pid > kill $make_id > if grep -q WARN /tmp/console.log; > then > echo $COUNTER > /tmp/counter > exit > fi > COUNTER=$(($COUNTER+1)) > done > > Above test shows that original kernel also warn about > "WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0" > > But I am not very sure about my results, so I still add a [RFC] to my subject line. > > Thank all of you for your guidance and encouragement ;-) > > Cheers > Zhouyi > -- > arch/powerpc/platforms/pseries/hotplug-cpu.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c > index e0a7ac5db15d..e47098f00da1 100644 > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c > @@ -64,10 +64,15 @@ static void pseries_cpu_offline_self(void) > > local_irq_disable(); > idle_task_exit(); > + /* prevent lockdep code from traveling RCU protected list > + * when we are offline. > + */ > + lockdep_off(); > if (xive_enabled()) > xive_teardown_cpu(); > else > xics_teardown_cpu(); > + lockdep_on(); > > unregister_slb_shadow(hwcpu); > rtas_stop_self(); > -- > 2.25.1
Thank Nick for reviewing my patch On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin <npiggin@gmail.com> wrote: > > On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > This is second version of my fix to PPC's "WARNING: suspicious RCU usage", > > I improved my fix under Paul E. McKenney's guidance: > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ > > > > During the cpu offlining, the sub functions of xive_teardown_cpu will > > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will > > travel RCU protected list, so "WARNING: suspicious RCU usage" will be > > triggered. > > > > Avoid lockdep when we are offline. > > I don't see how this is safe. If RCU is no longer watching the CPU then > the memory it is accessing here could be concurrently freed. I think the > warning is valid. Agree > > powerpc's problem is that cpuhp_report_idle_dead() is called before > arch_cpu_idle_dead(), so it must not rely on any RCU protection there. > I would say xive cleanup just needs to be done earlier. I wonder why it > is not done in __cpu_disable or thereabouts, that's where the interrupt > controller is supposed to be stopped. Yes, I learn flowing events sequence from kgdb debugging __cpu_disable -> pseries_cpu_disable -> set_cpu_online(cpu, false) = leads to => do_idle: if (cpu_is_offline(cpu) -> arch_cpu_idle_dead so xive cleanup should be done in pseries_cpu_disable. But as a beginner, I afraid that I am incompetent to do above sophisticated work without error although I am very like to, Could any expert do this for us? Thanks a lot Cheers Zhouyi > > Thanks, > Nick > > > > > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> > > --- > > Dear PPC and RCU developers > > > > I found this bug when trying to do rcutorture tests in ppc VM of > > Open Source Lab of Oregon State University > > > > console.log report following bug: > > [ 37.635545][ T0] WARNING: suspicious RCU usage^M > > [ 37.636409][ T0] 6.0.0-rc4-next-20220907-dirty #8 Not tainted^M > > [ 37.637575][ T0] -----------------------------^M > > [ 37.638306][ T0] kernel/locking/lockdep.c:3723 RCU-list traversed in non-reader section!!^M > > [ 37.639651][ T0] ^M > > [ 37.639651][ T0] other info that might help us debug this:^M > > [ 37.639651][ T0] ^M > > [ 37.641381][ T0] ^M > > [ 37.641381][ T0] RCU used illegally from offline CPU!^M > > [ 37.641381][ T0] rcu_scheduler_active = 2, debug_locks = 1^M > > [ 37.667170][ T0] no locks held by swapper/6/0.^M > > [ 37.668328][ T0] ^M > > [ 37.668328][ T0] stack backtrace:^M > > [ 37.669995][ T0] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 6.0.0-rc4-next-20220907-dirty #8^M > > [ 37.672777][ T0] Call Trace:^M > > [ 37.673729][ T0] [c000000004653920] [c00000000097f9b4] dump_stack_lvl+0x98/0xe0 (unreliable)^M > > [ 37.678579][ T0] [c000000004653960] [c0000000001f2eb8] lockdep_rcu_suspicious+0x148/0x16c^M > > [ 37.680425][ T0] [c0000000046539f0] [c0000000001ed9b4] __lock_acquire+0x10f4/0x26e0^M > > [ 37.682450][ T0] [c000000004653b30] [c0000000001efc2c] lock_acquire+0x12c/0x420^M > > [ 37.684113][ T0] [c000000004653c20] [c0000000010d704c] _raw_spin_lock_irqsave+0x6c/0xc0^M > > [ 37.686154][ T0] [c000000004653c60] [c0000000000c7b4c] xive_spapr_put_ipi+0xcc/0x150^M > > [ 37.687879][ T0] [c000000004653ca0] [c0000000010c72a8] xive_cleanup_cpu_ipi+0xc8/0xf0^M > > [ 37.689856][ T0] [c000000004653cf0] [c0000000010c7370] xive_teardown_cpu+0xa0/0xf0^M > > [ 37.691877][ T0] [c000000004653d30] [c0000000000fba5c] pseries_cpu_offline_self+0x5c/0x100^M > > [ 37.693882][ T0] [c000000004653da0] [c00000000005d2c4] arch_cpu_idle_dead+0x44/0x60^M > > [ 37.695739][ T0] [c000000004653dc0] [c0000000001c740c] do_idle+0x16c/0x3d0^M > > [ 37.697536][ T0] [c000000004653e70] [c0000000001c7a1c] cpu_startup_entry+0x3c/0x40^M > > [ 37.699694][ T0] [c000000004653ea0] [c00000000005ca20] start_secondary+0x6c0/0xb50^M > > [ 37.701742][ T0] [c000000004653f90] [c00000000000d054] start_secondary_prolog+0x10/0x14^M > > > > > > Tested on PPC VM of Open Source Lab of Oregon State University. > > Test results show that although "WARNING: suspicious RCU usage" has gone, > > and there are less "BUG: soft lockup" reports than the original kernel > > (9 vs 13), which sounds good ;-) > > > > But after my modification, results-rcutorture-kasan/SRCU-P/console.log.diags > > shows a new warning: > > [ 222.289242][ T110] WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0 > > > > I guess above new warning also exits in original kernel, so I write a tiny test script as follows: > > > > #!/bin/sh > > > > COUNTER=0 > > while [ $COUNTER -lt 1000 ] ; do > > qemu-system-ppc64 -nographic -smp cores=8,threads=1 -net none -M pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log -m 2G -kernel /tmp/vmlinux -append "debug_boot_weak_hash panic=-1 console=ttyS0 rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 rcutorture.torture_type=srcud rcupdate.rcu_self_test=1 rcutorture.fwd_progress=3 srcutree.big_cpu_lim=5 rcutorture.onoff_interval=1000 rcutorture.onoff_holdoff=30 rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15 rcutorture.shutdown_secs=420 rcutorture.test_no_idle_hz=1 rcutorture.verbose=1"& > > qemu_pid=$! > > cd ~/next1/linux-next > > make clean > > #I use "make vmlinux -j 8" to create heavy background jitter > > make vmlinux -j 8 > /dev/null 2>&1 > > make_pid=$! > > wait $qemu_pid > > kill $qemu_pid > > kill $make_id > > if grep -q WARN /tmp/console.log; > > then > > echo $COUNTER > /tmp/counter > > exit > > fi > > COUNTER=$(($COUNTER+1)) > > done > > > > Above test shows that original kernel also warn about > > "WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0" > > > > But I am not very sure about my results, so I still add a [RFC] to my subject line. > > > > Thank all of you for your guidance and encouragement ;-) > > > > Cheers > > Zhouyi > > -- > > arch/powerpc/platforms/pseries/hotplug-cpu.c | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c > > index e0a7ac5db15d..e47098f00da1 100644 > > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c > > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c > > @@ -64,10 +64,15 @@ static void pseries_cpu_offline_self(void) > > > > local_irq_disable(); > > idle_task_exit(); > > + /* prevent lockdep code from traveling RCU protected list > > + * when we are offline. > > + */ > > + lockdep_off(); > > if (xive_enabled()) > > xive_teardown_cpu(); > > else > > xics_teardown_cpu(); > > + lockdep_on(); > > > > unregister_slb_shadow(hwcpu); > > rtas_stop_self(); > > -- > > 2.25.1 >
On Wed Sep 28, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > Thank Nick for reviewing my patch > > On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin <npiggin@gmail.com> wrote: > > > > On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > > This is second version of my fix to PPC's "WARNING: suspicious RCU usage", > > > I improved my fix under Paul E. McKenney's guidance: > > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ > > > > > > During the cpu offlining, the sub functions of xive_teardown_cpu will > > > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will > > > travel RCU protected list, so "WARNING: suspicious RCU usage" will be > > > triggered. > > > > > > Avoid lockdep when we are offline. > > > > I don't see how this is safe. If RCU is no longer watching the CPU then > > the memory it is accessing here could be concurrently freed. I think the > > warning is valid. > Agree > > > > powerpc's problem is that cpuhp_report_idle_dead() is called before > > arch_cpu_idle_dead(), so it must not rely on any RCU protection there. > > I would say xive cleanup just needs to be done earlier. I wonder why it > > is not done in __cpu_disable or thereabouts, that's where the interrupt > > controller is supposed to be stopped. > Yes, I learn flowing events sequence from kgdb debugging > __cpu_disable -> pseries_cpu_disable -> set_cpu_online(cpu, false) = > leads to => do_idle: if (cpu_is_offline(cpu) -> arch_cpu_idle_dead > so xive cleanup should be done in pseries_cpu_disable. It's a good catch and a reasonable approach to the problem. > But as a beginner, I afraid that I am incompetent to do above > sophisticated work without error although I am very like to, > Could any expert do this for us? This will be difficult for anybody, it's tricky code. I'm not an expert at it. It looks like the interrupt controller disable split has been there since long before xive. I would try just move them together than see if that works. Documentation/core-api/cpu_hotplug.rst says that __cpu_disable should shut down the interrupt handler. So if there is a complication it would probably be from powerpc-specific CPU hotplug or interrupt code. Thanks, Nick
On Wed, Sep 28, 2022 at 10:51 AM Nicholas Piggin <npiggin@gmail.com> wrote: > > On Wed Sep 28, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > Thank Nick for reviewing my patch > > > > On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin <npiggin@gmail.com> wrote: > > > > > > On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > > > This is second version of my fix to PPC's "WARNING: suspicious RCU usage", > > > > I improved my fix under Paul E. McKenney's guidance: > > > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ > > > > > > > > During the cpu offlining, the sub functions of xive_teardown_cpu will > > > > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will > > > > travel RCU protected list, so "WARNING: suspicious RCU usage" will be > > > > triggered. > > > > > > > > Avoid lockdep when we are offline. > > > > > > I don't see how this is safe. If RCU is no longer watching the CPU then > > > the memory it is accessing here could be concurrently freed. I think the > > > warning is valid. > > Agree > > > > > > powerpc's problem is that cpuhp_report_idle_dead() is called before > > > arch_cpu_idle_dead(), so it must not rely on any RCU protection there. > > > I would say xive cleanup just needs to be done earlier. I wonder why it > > > is not done in __cpu_disable or thereabouts, that's where the interrupt > > > controller is supposed to be stopped. > > Yes, I learn flowing events sequence from kgdb debugging > > __cpu_disable -> pseries_cpu_disable -> set_cpu_online(cpu, false) = > > leads to => do_idle: if (cpu_is_offline(cpu) -> arch_cpu_idle_dead > > so xive cleanup should be done in pseries_cpu_disable. > > It's a good catch and a reasonable approach to the problem. Thank Nick for your encouragement ;-) > > > But as a beginner, I afraid that I am incompetent to do above > > sophisticated work without error although I am very like to, > > Could any expert do this for us? > > This will be difficult for anybody, it's tricky code. I'm not an > expert at it. > > It looks like the interrupt controller disable split has been there > since long before xive. I would try just move them together than see > if that works. Yes, I use "git blame" (I learned "git blame" from Paul E. McKenny ;-) ) to see the same. and anticipate your great works! > > Documentation/core-api/cpu_hotplug.rst says that __cpu_disable should > shut down the interrupt handler. So if there is a complication it > would probably be from powerpc-specific CPU hotplug or interrupt > code. Thank Nick for your guidance! I studied Documentation/core-api/cpu_hotplug.rst this morning. I also found X86 shut down the interrupt handler in __cpu_disable according to above document. Many Thanks Zhouyi > > Thanks, > Nick >
On Mon, Oct 10, 2022 at 11:49 AM Nicholas Piggin <npiggin@gmail.com> wrote: > > On Thu Sep 29, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > On Wed, Sep 28, 2022 at 10:51 AM Nicholas Piggin <npiggin@gmail.com> wrote: > > > > > > On Wed Sep 28, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > > > Thank Nick for reviewing my patch > > > > > > > > On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin <npiggin@gmail.com> wrote: > > > > > > > > > > On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > > > > > This is second version of my fix to PPC's "WARNING: suspicious RCU usage", > > > > > > I improved my fix under Paul E. McKenney's guidance: > > > > > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ > > > > > > > > > > > > During the cpu offlining, the sub functions of xive_teardown_cpu will > > > > > > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will > > > > > > travel RCU protected list, so "WARNING: suspicious RCU usage" will be > > > > > > triggered. > > > > > > > > > > > > Avoid lockdep when we are offline. > > > > > > > > > > I don't see how this is safe. If RCU is no longer watching the CPU then > > > > > the memory it is accessing here could be concurrently freed. I think the > > > > > warning is valid. > > > > Agree > > > > > > > > > > powerpc's problem is that cpuhp_report_idle_dead() is called before > > > > > arch_cpu_idle_dead(), so it must not rely on any RCU protection there. > > > > > I would say xive cleanup just needs to be done earlier. I wonder why it > > > > > is not done in __cpu_disable or thereabouts, that's where the interrupt > > > > > controller is supposed to be stopped. > > > > Yes, I learn flowing events sequence from kgdb debugging > > > > __cpu_disable -> pseries_cpu_disable -> set_cpu_online(cpu, false) = > > > > leads to => do_idle: if (cpu_is_offline(cpu) -> arch_cpu_idle_dead > > > > so xive cleanup should be done in pseries_cpu_disable. > > > > > > It's a good catch and a reasonable approach to the problem. > > Thank Nick for your encouragement ;-) > > > > > > > But as a beginner, I afraid that I am incompetent to do above > > > > sophisticated work without error although I am very like to, > > > > Could any expert do this for us? > > > > > > This will be difficult for anybody, it's tricky code. I'm not an > > > expert at it. > > > > > > It looks like the interrupt controller disable split has been there > > > since long before xive. I would try just move them together than see > > > if that works. > > Yes, I use "git blame" (I learned "git blame" from Paul E. McKenny ;-) > > ) to see the same. > > and anticipate your great works! > > I was thinking you could try it and see if it works and what you find. > If you are interested and have time to look into it? I am interested! and I have time ;-) Thank Nick for your trust in me! I am going to submit my babyish work in about a month (counting the rcutoture tests time), and thank you in advance for your patience. Cheers Zhouyi > > Thanks, > Nick
On Thu Sep 29, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > On Wed, Sep 28, 2022 at 10:51 AM Nicholas Piggin <npiggin@gmail.com> wrote: > > > > On Wed Sep 28, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > > Thank Nick for reviewing my patch > > > > > > On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin <npiggin@gmail.com> wrote: > > > > > > > > On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote: > > > > > This is second version of my fix to PPC's "WARNING: suspicious RCU usage", > > > > > I improved my fix under Paul E. McKenney's guidance: > > > > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ > > > > > > > > > > During the cpu offlining, the sub functions of xive_teardown_cpu will > > > > > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will > > > > > travel RCU protected list, so "WARNING: suspicious RCU usage" will be > > > > > triggered. > > > > > > > > > > Avoid lockdep when we are offline. > > > > > > > > I don't see how this is safe. If RCU is no longer watching the CPU then > > > > the memory it is accessing here could be concurrently freed. I think the > > > > warning is valid. > > > Agree > > > > > > > > powerpc's problem is that cpuhp_report_idle_dead() is called before > > > > arch_cpu_idle_dead(), so it must not rely on any RCU protection there. > > > > I would say xive cleanup just needs to be done earlier. I wonder why it > > > > is not done in __cpu_disable or thereabouts, that's where the interrupt > > > > controller is supposed to be stopped. > > > Yes, I learn flowing events sequence from kgdb debugging > > > __cpu_disable -> pseries_cpu_disable -> set_cpu_online(cpu, false) = > > > leads to => do_idle: if (cpu_is_offline(cpu) -> arch_cpu_idle_dead > > > so xive cleanup should be done in pseries_cpu_disable. > > > > It's a good catch and a reasonable approach to the problem. > Thank Nick for your encouragement ;-) > > > > > But as a beginner, I afraid that I am incompetent to do above > > > sophisticated work without error although I am very like to, > > > Could any expert do this for us? > > > > This will be difficult for anybody, it's tricky code. I'm not an > > expert at it. > > > > It looks like the interrupt controller disable split has been there > > since long before xive. I would try just move them together than see > > if that works. > Yes, I use "git blame" (I learned "git blame" from Paul E. McKenny ;-) > ) to see the same. > and anticipate your great works! I was thinking you could try it and see if it works and what you find. If you are interested and have time to look into it? Thanks, Nick
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c index e0a7ac5db15d..e47098f00da1 100644 --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c @@ -64,10 +64,15 @@ static void pseries_cpu_offline_self(void) local_irq_disable(); idle_task_exit(); + /* prevent lockdep code from traveling RCU protected list + * when we are offline. + */ + lockdep_off(); if (xive_enabled()) xive_teardown_cpu(); else xics_teardown_cpu(); + lockdep_on(); unregister_slb_shadow(hwcpu); rtas_stop_self();
This is second version of my fix to PPC's "WARNING: suspicious RCU usage", I improved my fix under Paul E. McKenney's guidance: Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/ During the cpu offlining, the sub functions of xive_teardown_cpu will call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will travel RCU protected list, so "WARNING: suspicious RCU usage" will be triggered. Avoid lockdep when we are offline. Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com> --- Dear PPC and RCU developers I found this bug when trying to do rcutorture tests in ppc VM of Open Source Lab of Oregon State University console.log report following bug: [ 37.635545][ T0] WARNING: suspicious RCU usage^M [ 37.636409][ T0] 6.0.0-rc4-next-20220907-dirty #8 Not tainted^M [ 37.637575][ T0] -----------------------------^M [ 37.638306][ T0] kernel/locking/lockdep.c:3723 RCU-list traversed in non-reader section!!^M [ 37.639651][ T0] ^M [ 37.639651][ T0] other info that might help us debug this:^M [ 37.639651][ T0] ^M [ 37.641381][ T0] ^M [ 37.641381][ T0] RCU used illegally from offline CPU!^M [ 37.641381][ T0] rcu_scheduler_active = 2, debug_locks = 1^M [ 37.667170][ T0] no locks held by swapper/6/0.^M [ 37.668328][ T0] ^M [ 37.668328][ T0] stack backtrace:^M [ 37.669995][ T0] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 6.0.0-rc4-next-20220907-dirty #8^M [ 37.672777][ T0] Call Trace:^M [ 37.673729][ T0] [c000000004653920] [c00000000097f9b4] dump_stack_lvl+0x98/0xe0 (unreliable)^M [ 37.678579][ T0] [c000000004653960] [c0000000001f2eb8] lockdep_rcu_suspicious+0x148/0x16c^M [ 37.680425][ T0] [c0000000046539f0] [c0000000001ed9b4] __lock_acquire+0x10f4/0x26e0^M [ 37.682450][ T0] [c000000004653b30] [c0000000001efc2c] lock_acquire+0x12c/0x420^M [ 37.684113][ T0] [c000000004653c20] [c0000000010d704c] _raw_spin_lock_irqsave+0x6c/0xc0^M [ 37.686154][ T0] [c000000004653c60] [c0000000000c7b4c] xive_spapr_put_ipi+0xcc/0x150^M [ 37.687879][ T0] [c000000004653ca0] [c0000000010c72a8] xive_cleanup_cpu_ipi+0xc8/0xf0^M [ 37.689856][ T0] [c000000004653cf0] [c0000000010c7370] xive_teardown_cpu+0xa0/0xf0^M [ 37.691877][ T0] [c000000004653d30] [c0000000000fba5c] pseries_cpu_offline_self+0x5c/0x100^M [ 37.693882][ T0] [c000000004653da0] [c00000000005d2c4] arch_cpu_idle_dead+0x44/0x60^M [ 37.695739][ T0] [c000000004653dc0] [c0000000001c740c] do_idle+0x16c/0x3d0^M [ 37.697536][ T0] [c000000004653e70] [c0000000001c7a1c] cpu_startup_entry+0x3c/0x40^M [ 37.699694][ T0] [c000000004653ea0] [c00000000005ca20] start_secondary+0x6c0/0xb50^M [ 37.701742][ T0] [c000000004653f90] [c00000000000d054] start_secondary_prolog+0x10/0x14^M Tested on PPC VM of Open Source Lab of Oregon State University. Test results show that although "WARNING: suspicious RCU usage" has gone, and there are less "BUG: soft lockup" reports than the original kernel (9 vs 13), which sounds good ;-) But after my modification, results-rcutorture-kasan/SRCU-P/console.log.diags shows a new warning: [ 222.289242][ T110] WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0 I guess above new warning also exits in original kernel, so I write a tiny test script as follows: #!/bin/sh COUNTER=0 while [ $COUNTER -lt 1000 ] ; do qemu-system-ppc64 -nographic -smp cores=8,threads=1 -net none -M pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log -m 2G -kernel /tmp/vmlinux -append "debug_boot_weak_hash panic=-1 console=ttyS0 rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 rcutorture.torture_type=srcud rcupdate.rcu_self_test=1 rcutorture.fwd_progress=3 srcutree.big_cpu_lim=5 rcutorture.onoff_interval=1000 rcutorture.onoff_holdoff=30 rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15 rcutorture.shutdown_secs=420 rcutorture.test_no_idle_hz=1 rcutorture.verbose=1"& qemu_pid=$! cd ~/next1/linux-next make clean #I use "make vmlinux -j 8" to create heavy background jitter make vmlinux -j 8 > /dev/null 2>&1 make_pid=$! wait $qemu_pid kill $qemu_pid kill $make_id if grep -q WARN /tmp/console.log; then echo $COUNTER > /tmp/counter exit fi COUNTER=$(($COUNTER+1)) done Above test shows that original kernel also warn about "WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0" But I am not very sure about my results, so I still add a [RFC] to my subject line. Thank all of you for your guidance and encouragement ;-) Cheers Zhouyi -- arch/powerpc/platforms/pseries/hotplug-cpu.c | 5 +++++ 1 file changed, 5 insertions(+)