* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
[not found] <20200904153231.11994-1-a.darwish@linutronix.de>
@ 2020-09-15 0:20 ` Qian Cai
2020-09-15 12:48 ` Boqun Feng
0 siblings, 1 reply; 9+ messages in thread
From: Qian Cai @ 2020-09-15 0:20 UTC (permalink / raw)
To: Ahmed S. Darwish, Peter Zijlstra, Ingo Molnar, Will Deacon
Cc: Thomas Gleixner, Sebastian A. Siewior, Paul E. McKenney,
Steven Rostedt, LKML, Stephen Rothwell, linux-next, Waiman Long,
Boqun Feng
On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> Hi,
>
> Changelog-v2
> ============
>
> - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> sequence counters with associated locks, instead of v1
> seqcount_LOCKTYPE_t.
>
> - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> property accessors.
>
> - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
>
> Cover letter (v1)
> =================
>
> https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de
>
> Preemption must be disabled before entering a sequence counter write
> side critical section. Otherwise the read side section can preempt the
> write side section and spin for the entire scheduler tick. If that
> reader belongs to a real-time scheduling class, it can spin forever and
> the kernel will livelock.
>
> Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> to higher latencies, and the write side sections will not be able to
> acquire locks which become sleeping locks (e.g. spinlock_t).
>
> To remain preemptible, while avoiding a possible livelock caused by the
> reader preempting the writer, use a different technique: let the reader
> detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> case, acquire then release the associated LOCKNAME writer serialization
> lock. This will allow any possibly-preempted writer to make progress
> until the end of its writer serialization lock critical section.
>
> Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
Reverting this patchset [1] from today's linux-next fixed a splat below. The
splat looks like a false positive anyway because the existing locking dependency
chains from the task #1 here:
&s->seqcount#2 ---> pidmap_lock
[ 528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
[ 528.078078][ T7867] lock_acquire+0x10c/0x560
[ 528.078089][ T7867] _raw_spin_lock_irqsave+0x64/0xb0
[ 528.078108][ T7867] free_pid+0x5c/0x160
free_pid at kernel/pid.c:131
[ 528.078127][ T7867] release_task.part.40+0x59c/0x7f0
__unhash_process at kernel/exit.c:76
(inlined by) __exit_signal at kernel/exit.c:147
(inlined by) release_task at kernel/exit.c:198
[ 528.078145][ T7867] do_exit+0x77c/0xda0
exit_notify at kernel/exit.c:679
(inlined by) do_exit at kernel/exit.c:826
[ 528.078163][ T7867] kthread+0x148/0x1d0
[ 528.078182][ T7867] ret_from_kernel_thread+0x5c/0x80
It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2
in read_mems_allowed_begin() is read_seqcount_begin(¤t->mems_allowed_seq),
so there should be no deadlock?
[1] git revert --no-edit 0c9794c8b678..1909760f5fc3
[ 528.077900][ T7867] WARNING: possible circular locking dependency detected
[ 528.077912][ T7867] 5.9.0-rc5-next-20200914 #1 Not tainted
[ 528.077921][ T7867] ------------------------------------------------------
[ 528.077931][ T7867] runc:[1:CHILD]/7867 is trying to acquire lock:
[ 528.077942][ T7867] c000001fce5570c8 (&s->seqcount#2){....}-{0:0}, at: __slab_alloc+0x34/0xf0
[ 528.077972][ T7867]
[ 528.077972][ T7867] but task is already holding lock:
[ 528.077983][ T7867] c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
[ 528.078009][ T7867]
[ 528.078009][ T7867] which lock already depends on the new lock.
[ 528.078009][ T7867]
[ 528.078031][ T7867]
[ 528.078031][ T7867] the existing dependency chain (in reverse order) is:
[ 528.078061][ T7867]
[ 528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
[ 528.078078][ T7867] lock_acquire+0x10c/0x560
[ 528.078089][ T7867] _raw_spin_lock_irqsave+0x64/0xb0
[ 528.078108][ T7867] free_pid+0x5c/0x160
free_pid at kernel/pid.c:131
[ 528.078127][ T7867] release_task.part.40+0x59c/0x7f0
__unhash_process at kernel/exit.c:76
(inlined by) __exit_signal at kernel/exit.c:147
(inlined by) release_task at kernel/exit.c:198
[ 528.078145][ T7867] do_exit+0x77c/0xda0
exit_notify at kernel/exit.c:679
(inlined by) do_exit at kernel/exit.c:826
[ 528.078163][ T7867] kthread+0x148/0x1d0
[ 528.078182][ T7867] ret_from_kernel_thread+0x5c/0x80
[ 528.078208][ T7867]
[ 528.078208][ T7867] -> #0 (&s->seqcount#2){....}-{0:0}:
[ 528.078241][ T7867] check_prevs_add+0x1c4/0x1120
check_prev_add at kernel/locking/lockdep.c:2820
(inlined by) check_prevs_add at kernel/locking/lockdep.c:2944
[ 528.078260][ T7867] __lock_acquire+0x176c/0x1c00
validate_chain at kernel/locking/lockdep.c:3562
(inlined by) __lock_acquire at kernel/locking/lockdep.c:4796
[ 528.078278][ T7867] lock_acquire+0x10c/0x560
[ 528.078297][ T7867] ___slab_alloc+0xa40/0xb40
seqcount_lockdep_reader_access at include/linux/seqlock.h:103
(inlined by) read_mems_allowed_begin at include/linux/cpuset.h:135
(inlined by) get_any_partial at mm/slub.c:2035
(inlined by) get_partial at mm/slub.c:2078
(inlined by) new_slab_objects at mm/slub.c:2577
(inlined by) ___slab_alloc at mm/slub.c:2745
[ 528.078324][ T7867] __slab_alloc+0x34/0xf0
[ 528.078342][ T7867] kmem_cache_alloc+0x2d4/0x470
[ 528.078362][ T7867] create_object+0x74/0x430
[ 528.078381][ T7867] slab_post_alloc_hook+0xa4/0x670
[ 528.078399][ T7867] kmem_cache_alloc+0x1b4/0x470
[ 528.078418][ T7867] radix_tree_node_alloc.constprop.19+0xe4/0x160
[ 528.078438][ T7867] idr_get_free+0x298/0x360
[ 528.078456][ T7867] idr_alloc_u32+0x84/0x130
[ 528.078474][ T7867] idr_alloc_cyclic+0x7c/0x150
[ 528.078493][ T7867] alloc_pid+0x27c/0x590
[ 528.078511][ T7867] copy_process+0xc90/0x1930
copy_process at kernel/fork.c:2104
[ 528.078529][ T7867] kernel_clone+0x120/0xa10
[ 528.078546][ T7867] __do_sys_clone+0x88/0xd0
[ 528.078565][ T7867] system_call_exception+0xf8/0x1d0
[ 528.078592][ T7867] system_call_common+0xe8/0x218
[ 528.078609][ T7867]
[ 528.078609][ T7867] other info that might help us debug this:
[ 528.078609][ T7867]
[ 528.078650][ T7867] Possible unsafe locking scenario:
[ 528.078650][ T7867]
[ 528.078670][ T7867] CPU0 CPU1
[ 528.078695][ T7867] ---- ----
[ 528.078713][ T7867] lock(pidmap_lock);
[ 528.078730][ T7867] lock(&s->seqcount#2);
[ 528.078751][ T7867] lock(pidmap_lock);
[ 528.078770][ T7867] lock(&s->seqcount#2);
[ 528.078788][ T7867]
[ 528.078788][ T7867] *** DEADLOCK ***
[ 528.078788][ T7867]
[ 528.078800][ T7867] 2 locks held by runc:[1:CHILD]/7867:
[ 528.078808][ T7867] #0: c000001ffea6f4f0 (lock#2){+.+.}-{2:2}, at: __radix_tree_preload+0x8/0x370
__radix_tree_preload at lib/radix-tree.c:322
[ 528.078844][ T7867] #1: c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
[ 528.078870][ T7867]
[ 528.078870][ T7867] stack backtrace:
[ 528.078890][ T7867] CPU: 46 PID: 7867 Comm: runc:[1:CHILD] Not tainted 5.9.0-rc5-next-20200914 #1
[ 528.078921][ T7867] Call Trace:
[ 528.078940][ T7867] [c000001ff07eefc0] [c00000000063f8c8] dump_stack+0xec/0x144 (unreliable)
[ 528.078964][ T7867] [c000001ff07ef000] [c00000000013f44c] print_circular_bug.isra.43+0x2dc/0x350
[ 528.078978][ T7867] [c000001ff07ef0a0] [c00000000013f640] check_noncircular+0x180/0x1b0
[ 528.079000][ T7867] [c000001ff07ef170] [c000000000140b84] check_prevs_add+0x1c4/0x1120
[ 528.079022][ T7867] [c000001ff07ef280] [c0000000001446ec] __lock_acquire+0x176c/0x1c00
[ 528.079043][ T7867] [c000001ff07ef3a0] [c00000000014578c] lock_acquire+0x10c/0x560
[ 528.079066][ T7867] [c000001ff07ef490] [c0000000003565f0] ___slab_alloc+0xa40/0xb40
[ 528.079079][ T7867] [c000001ff07ef590] [c000000000356724] __slab_alloc+0x34/0xf0
[ 528.079100][ T7867] [c000001ff07ef5e0] [c000000000356ab4] kmem_cache_alloc+0x2d4/0x470
[ 528.079122][ T7867] [c000001ff07ef670] [c000000000397e14] create_object+0x74/0x430
[ 528.079144][ T7867] [c000001ff07ef720] [c000000000351944] slab_post_alloc_hook+0xa4/0x670
[ 528.079165][ T7867] [c000001ff07ef7e0] [c000000000356994] kmem_cache_alloc+0x1b4/0x470
[ 528.079187][ T7867] [c000001ff07ef870] [c00000000064e004] radix_tree_node_alloc.constprop.19+0xe4/0x160
radix_tree_node_alloc at lib/radix-tree.c:252
[ 528.079219][ T7867] [c000001ff07ef8e0] [c00000000064f2b8] idr_get_free+0x298/0x360
idr_get_free at lib/radix-tree.c:1507
[ 528.079249][ T7867] [c000001ff07ef970] [c000000000645db4] idr_alloc_u32+0x84/0x130
idr_alloc_u32 at lib/idr.c:46 (discriminator 4)
[ 528.079271][ T7867] [c000001ff07ef9e0] [c000000000645f8c] idr_alloc_cyclic+0x7c/0x150
idr_alloc_cyclic at lib/idr.c:126 (discriminator 1)
[ 528.079301][ T7867] [c000001ff07efa40] [c0000000000e48ac] alloc_pid+0x27c/0x590
[ 528.079342][ T7867] [c000001ff07efb20] [c0000000000acc60] copy_process+0xc90/0x1930
[ 528.079404][ T7867] [c000001ff07efc40] [c0000000000adc00] kernel_clone+0x120/0xa10
[ 528.079499][ T7867] [c000001ff07efd00] [c0000000000ae578] __do_sys_clone+0x88/0xd0
[ 528.079579][ T7867] [c000001ff07efdc0] [c000000000029c48] system_call_exception+0xf8/0x1d0
[ 528.079691][ T7867] [c000001ff07efe20] [c00000000000d0a8] system_call_common+0xe8/0x218
>
> 8<--------------
>
> Ahmed S. Darwish (5):
> seqlock: seqcount_LOCKNAME_t: Standardize naming convention
> seqlock: Use unique prefix for seqcount_t property accessors
> seqlock: seqcount_t: Implement all read APIs as statement expressions
> seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
> seqlock: PREEMPT_RT: Do not starve seqlock_t writers
>
> include/linux/seqlock.h | 281 ++++++++++++++++++++++++----------------
> 1 file changed, 167 insertions(+), 114 deletions(-)
>
> base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
> --
> 2.28.0
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-15 0:20 ` [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support Qian Cai
@ 2020-09-15 12:48 ` Boqun Feng
2020-09-15 13:10 ` Boqun Feng
2020-09-15 14:30 ` peterz
0 siblings, 2 replies; 9+ messages in thread
From: Boqun Feng @ 2020-09-15 12:48 UTC (permalink / raw)
To: Qian Cai
Cc: Ahmed S. Darwish, Peter Zijlstra, Ingo Molnar, Will Deacon,
Thomas Gleixner, Sebastian A. Siewior, Paul E. McKenney,
Steven Rostedt, LKML, Stephen Rothwell, linux-next, Waiman Long
On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote:
> On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> > Hi,
> >
> > Changelog-v2
> > ============
> >
> > - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> > sequence counters with associated locks, instead of v1
> > seqcount_LOCKTYPE_t.
> >
> > - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> > property accessors.
> >
> > - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> > non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> >
> > Cover letter (v1)
> > =================
> >
> > https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de
> >
> > Preemption must be disabled before entering a sequence counter write
> > side critical section. Otherwise the read side section can preempt the
> > write side section and spin for the entire scheduler tick. If that
> > reader belongs to a real-time scheduling class, it can spin forever and
> > the kernel will livelock.
> >
> > Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> > to higher latencies, and the write side sections will not be able to
> > acquire locks which become sleeping locks (e.g. spinlock_t).
> >
> > To remain preemptible, while avoiding a possible livelock caused by the
> > reader preempting the writer, use a different technique: let the reader
> > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> > case, acquire then release the associated LOCKNAME writer serialization
> > lock. This will allow any possibly-preempted writer to make progress
> > until the end of its writer serialization lock critical section.
> >
> > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
>
> Reverting this patchset [1] from today's linux-next fixed a splat below. The
> splat looks like a false positive anyway because the existing locking dependency
> chains from the task #1 here:
>
> &s->seqcount#2 ---> pidmap_lock
>
> [ 528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
> [ 528.078078][ T7867] lock_acquire+0x10c/0x560
> [ 528.078089][ T7867] _raw_spin_lock_irqsave+0x64/0xb0
> [ 528.078108][ T7867] free_pid+0x5c/0x160
> free_pid at kernel/pid.c:131
> [ 528.078127][ T7867] release_task.part.40+0x59c/0x7f0
> __unhash_process at kernel/exit.c:76
> (inlined by) __exit_signal at kernel/exit.c:147
> (inlined by) release_task at kernel/exit.c:198
> [ 528.078145][ T7867] do_exit+0x77c/0xda0
> exit_notify at kernel/exit.c:679
> (inlined by) do_exit at kernel/exit.c:826
> [ 528.078163][ T7867] kthread+0x148/0x1d0
> [ 528.078182][ T7867] ret_from_kernel_thread+0x5c/0x80
>
> It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2
> in read_mems_allowed_begin() is read_seqcount_begin(¤t->mems_allowed_seq),
> so there should be no deadlock?
>
I think this happened because seqcount_##lockname##_init() is defined at
function rather than macro, so when the seqcount_init() gets expand in
that function, the lock_class_key of seqcount will be a static variable
of seqcount_##lockname##_init() function, as a result, all
seqcount_##lockname##_t in the same compile unit (in this case it's
kernel/fork.c) share the same lock class key, and lockdep thought they
are the same lock ;-)
Regards,
Boqun
> [1] git revert --no-edit 0c9794c8b678..1909760f5fc3
>
> [ 528.077900][ T7867] WARNING: possible circular locking dependency detected
> [ 528.077912][ T7867] 5.9.0-rc5-next-20200914 #1 Not tainted
> [ 528.077921][ T7867] ------------------------------------------------------
> [ 528.077931][ T7867] runc:[1:CHILD]/7867 is trying to acquire lock:
> [ 528.077942][ T7867] c000001fce5570c8 (&s->seqcount#2){....}-{0:0}, at: __slab_alloc+0x34/0xf0
> [ 528.077972][ T7867]
> [ 528.077972][ T7867] but task is already holding lock:
> [ 528.077983][ T7867] c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
> [ 528.078009][ T7867]
> [ 528.078009][ T7867] which lock already depends on the new lock.
> [ 528.078009][ T7867]
> [ 528.078031][ T7867]
> [ 528.078031][ T7867] the existing dependency chain (in reverse order) is:
> [ 528.078061][ T7867]
> [ 528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
> [ 528.078078][ T7867] lock_acquire+0x10c/0x560
> [ 528.078089][ T7867] _raw_spin_lock_irqsave+0x64/0xb0
> [ 528.078108][ T7867] free_pid+0x5c/0x160
> free_pid at kernel/pid.c:131
> [ 528.078127][ T7867] release_task.part.40+0x59c/0x7f0
> __unhash_process at kernel/exit.c:76
> (inlined by) __exit_signal at kernel/exit.c:147
> (inlined by) release_task at kernel/exit.c:198
> [ 528.078145][ T7867] do_exit+0x77c/0xda0
> exit_notify at kernel/exit.c:679
> (inlined by) do_exit at kernel/exit.c:826
> [ 528.078163][ T7867] kthread+0x148/0x1d0
> [ 528.078182][ T7867] ret_from_kernel_thread+0x5c/0x80
> [ 528.078208][ T7867]
> [ 528.078208][ T7867] -> #0 (&s->seqcount#2){....}-{0:0}:
> [ 528.078241][ T7867] check_prevs_add+0x1c4/0x1120
> check_prev_add at kernel/locking/lockdep.c:2820
> (inlined by) check_prevs_add at kernel/locking/lockdep.c:2944
> [ 528.078260][ T7867] __lock_acquire+0x176c/0x1c00
> validate_chain at kernel/locking/lockdep.c:3562
> (inlined by) __lock_acquire at kernel/locking/lockdep.c:4796
> [ 528.078278][ T7867] lock_acquire+0x10c/0x560
> [ 528.078297][ T7867] ___slab_alloc+0xa40/0xb40
> seqcount_lockdep_reader_access at include/linux/seqlock.h:103
> (inlined by) read_mems_allowed_begin at include/linux/cpuset.h:135
> (inlined by) get_any_partial at mm/slub.c:2035
> (inlined by) get_partial at mm/slub.c:2078
> (inlined by) new_slab_objects at mm/slub.c:2577
> (inlined by) ___slab_alloc at mm/slub.c:2745
> [ 528.078324][ T7867] __slab_alloc+0x34/0xf0
> [ 528.078342][ T7867] kmem_cache_alloc+0x2d4/0x470
> [ 528.078362][ T7867] create_object+0x74/0x430
> [ 528.078381][ T7867] slab_post_alloc_hook+0xa4/0x670
> [ 528.078399][ T7867] kmem_cache_alloc+0x1b4/0x470
> [ 528.078418][ T7867] radix_tree_node_alloc.constprop.19+0xe4/0x160
> [ 528.078438][ T7867] idr_get_free+0x298/0x360
> [ 528.078456][ T7867] idr_alloc_u32+0x84/0x130
> [ 528.078474][ T7867] idr_alloc_cyclic+0x7c/0x150
> [ 528.078493][ T7867] alloc_pid+0x27c/0x590
> [ 528.078511][ T7867] copy_process+0xc90/0x1930
> copy_process at kernel/fork.c:2104
> [ 528.078529][ T7867] kernel_clone+0x120/0xa10
> [ 528.078546][ T7867] __do_sys_clone+0x88/0xd0
> [ 528.078565][ T7867] system_call_exception+0xf8/0x1d0
> [ 528.078592][ T7867] system_call_common+0xe8/0x218
> [ 528.078609][ T7867]
> [ 528.078609][ T7867] other info that might help us debug this:
> [ 528.078609][ T7867]
> [ 528.078650][ T7867] Possible unsafe locking scenario:
> [ 528.078650][ T7867]
> [ 528.078670][ T7867] CPU0 CPU1
> [ 528.078695][ T7867] ---- ----
> [ 528.078713][ T7867] lock(pidmap_lock);
> [ 528.078730][ T7867] lock(&s->seqcount#2);
> [ 528.078751][ T7867] lock(pidmap_lock);
> [ 528.078770][ T7867] lock(&s->seqcount#2);
> [ 528.078788][ T7867]
> [ 528.078788][ T7867] *** DEADLOCK ***
> [ 528.078788][ T7867]
> [ 528.078800][ T7867] 2 locks held by runc:[1:CHILD]/7867:
> [ 528.078808][ T7867] #0: c000001ffea6f4f0 (lock#2){+.+.}-{2:2}, at: __radix_tree_preload+0x8/0x370
> __radix_tree_preload at lib/radix-tree.c:322
> [ 528.078844][ T7867] #1: c0000000056b0198 (pidmap_lock){....}-{2:2}, at: alloc_pid+0x258/0x590
> [ 528.078870][ T7867]
> [ 528.078870][ T7867] stack backtrace:
> [ 528.078890][ T7867] CPU: 46 PID: 7867 Comm: runc:[1:CHILD] Not tainted 5.9.0-rc5-next-20200914 #1
> [ 528.078921][ T7867] Call Trace:
> [ 528.078940][ T7867] [c000001ff07eefc0] [c00000000063f8c8] dump_stack+0xec/0x144 (unreliable)
> [ 528.078964][ T7867] [c000001ff07ef000] [c00000000013f44c] print_circular_bug.isra.43+0x2dc/0x350
> [ 528.078978][ T7867] [c000001ff07ef0a0] [c00000000013f640] check_noncircular+0x180/0x1b0
> [ 528.079000][ T7867] [c000001ff07ef170] [c000000000140b84] check_prevs_add+0x1c4/0x1120
> [ 528.079022][ T7867] [c000001ff07ef280] [c0000000001446ec] __lock_acquire+0x176c/0x1c00
> [ 528.079043][ T7867] [c000001ff07ef3a0] [c00000000014578c] lock_acquire+0x10c/0x560
> [ 528.079066][ T7867] [c000001ff07ef490] [c0000000003565f0] ___slab_alloc+0xa40/0xb40
> [ 528.079079][ T7867] [c000001ff07ef590] [c000000000356724] __slab_alloc+0x34/0xf0
> [ 528.079100][ T7867] [c000001ff07ef5e0] [c000000000356ab4] kmem_cache_alloc+0x2d4/0x470
> [ 528.079122][ T7867] [c000001ff07ef670] [c000000000397e14] create_object+0x74/0x430
> [ 528.079144][ T7867] [c000001ff07ef720] [c000000000351944] slab_post_alloc_hook+0xa4/0x670
> [ 528.079165][ T7867] [c000001ff07ef7e0] [c000000000356994] kmem_cache_alloc+0x1b4/0x470
> [ 528.079187][ T7867] [c000001ff07ef870] [c00000000064e004] radix_tree_node_alloc.constprop.19+0xe4/0x160
> radix_tree_node_alloc at lib/radix-tree.c:252
> [ 528.079219][ T7867] [c000001ff07ef8e0] [c00000000064f2b8] idr_get_free+0x298/0x360
> idr_get_free at lib/radix-tree.c:1507
> [ 528.079249][ T7867] [c000001ff07ef970] [c000000000645db4] idr_alloc_u32+0x84/0x130
> idr_alloc_u32 at lib/idr.c:46 (discriminator 4)
> [ 528.079271][ T7867] [c000001ff07ef9e0] [c000000000645f8c] idr_alloc_cyclic+0x7c/0x150
> idr_alloc_cyclic at lib/idr.c:126 (discriminator 1)
> [ 528.079301][ T7867] [c000001ff07efa40] [c0000000000e48ac] alloc_pid+0x27c/0x590
> [ 528.079342][ T7867] [c000001ff07efb20] [c0000000000acc60] copy_process+0xc90/0x1930
> [ 528.079404][ T7867] [c000001ff07efc40] [c0000000000adc00] kernel_clone+0x120/0xa10
> [ 528.079499][ T7867] [c000001ff07efd00] [c0000000000ae578] __do_sys_clone+0x88/0xd0
> [ 528.079579][ T7867] [c000001ff07efdc0] [c000000000029c48] system_call_exception+0xf8/0x1d0
> [ 528.079691][ T7867] [c000001ff07efe20] [c00000000000d0a8] system_call_common+0xe8/0x218
>
> >
> > 8<--------------
> >
> > Ahmed S. Darwish (5):
> > seqlock: seqcount_LOCKNAME_t: Standardize naming convention
> > seqlock: Use unique prefix for seqcount_t property accessors
> > seqlock: seqcount_t: Implement all read APIs as statement expressions
> > seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
> > seqlock: PREEMPT_RT: Do not starve seqlock_t writers
> >
> > include/linux/seqlock.h | 281 ++++++++++++++++++++++++----------------
> > 1 file changed, 167 insertions(+), 114 deletions(-)
> >
> > base-commit: f75aef392f869018f78cfedf3c320a6b3fcfda6b
> > --
> > 2.28.0
>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-15 12:48 ` Boqun Feng
@ 2020-09-15 13:10 ` Boqun Feng
2020-09-15 14:30 ` peterz
1 sibling, 0 replies; 9+ messages in thread
From: Boqun Feng @ 2020-09-15 13:10 UTC (permalink / raw)
To: Qian Cai
Cc: Ahmed S. Darwish, Peter Zijlstra, Ingo Molnar, Will Deacon,
Thomas Gleixner, Sebastian A. Siewior, Paul E. McKenney,
Steven Rostedt, LKML, Stephen Rothwell, linux-next, Waiman Long
On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> On Mon, Sep 14, 2020 at 08:20:53PM -0400, Qian Cai wrote:
> > On Fri, 2020-09-04 at 17:32 +0200, Ahmed S. Darwish wrote:
> > > Hi,
> > >
> > > Changelog-v2
> > > ============
> > >
> > > - Standardize on seqcount_LOCKNAME_t as the canonical reference for
> > > sequence counters with associated locks, instead of v1
> > > seqcount_LOCKTYPE_t.
> > >
> > > - Use unique prefix "seqprop_*" for all seqcount_t/seqcount_LOCKNAME_t
> > > property accessors.
> > >
> > > - Touch-up the lock-unlock rationale for more clarity. Enforce writer
> > > non-preemitiblity using "__seq_enforce_writer_non_preemptibility()".
> > >
> > > Cover letter (v1)
> > > =================
> > >
> > > https://lkml.kernel.org/r/20200828010710.5407-1-a.darwish@linutronix.de
> > >
> > > Preemption must be disabled before entering a sequence counter write
> > > side critical section. Otherwise the read side section can preempt the
> > > write side section and spin for the entire scheduler tick. If that
> > > reader belongs to a real-time scheduling class, it can spin forever and
> > > the kernel will livelock.
> > >
> > > Disabling preemption cannot be done for PREEMPT_RT though: it can lead
> > > to higher latencies, and the write side sections will not be able to
> > > acquire locks which become sleeping locks (e.g. spinlock_t).
> > >
> > > To remain preemptible, while avoiding a possible livelock caused by the
> > > reader preempting the writer, use a different technique: let the reader
> > > detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
> > > case, acquire then release the associated LOCKNAME writer serialization
> > > lock. This will allow any possibly-preempted writer to make progress
> > > until the end of its writer serialization lock critical section.
> > >
> > > Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
> > > an associated (PREEMPT_RT) sleeping lock, and for seqlock_t.
> >
> > Reverting this patchset [1] from today's linux-next fixed a splat below. The
> > splat looks like a false positive anyway because the existing locking dependency
> > chains from the task #1 here:
> >
> > &s->seqcount#2 ---> pidmap_lock
> >
> > [ 528.078061][ T7867] -> #1 (pidmap_lock){....}-{2:2}:
> > [ 528.078078][ T7867] lock_acquire+0x10c/0x560
> > [ 528.078089][ T7867] _raw_spin_lock_irqsave+0x64/0xb0
> > [ 528.078108][ T7867] free_pid+0x5c/0x160
> > free_pid at kernel/pid.c:131
> > [ 528.078127][ T7867] release_task.part.40+0x59c/0x7f0
> > __unhash_process at kernel/exit.c:76
> > (inlined by) __exit_signal at kernel/exit.c:147
> > (inlined by) release_task at kernel/exit.c:198
> > [ 528.078145][ T7867] do_exit+0x77c/0xda0
> > exit_notify at kernel/exit.c:679
> > (inlined by) do_exit at kernel/exit.c:826
> > [ 528.078163][ T7867] kthread+0x148/0x1d0
> > [ 528.078182][ T7867] ret_from_kernel_thread+0x5c/0x80
> >
> > It is write_seqlock(&sig->stats_lock) in __exit_signal(), but the &s->seqcount#2
> > in read_mems_allowed_begin() is read_seqcount_begin(¤t->mems_allowed_seq),
> > so there should be no deadlock?
> >
>
> I think this happened because seqcount_##lockname##_init() is defined at
> function rather than macro, so when the seqcount_init() gets expand in
> that function, the lock_class_key of seqcount will be a static variable
> of seqcount_##lockname##_init() function, as a result, all
> seqcount_##lockname##_t in the same compile unit (in this case it's
> kernel/fork.c) share the same lock class key, and lockdep thought they
> are the same lock ;-)
>
Don't know how to fix this properly, but below is an ugly attemption,
only build test, just food for thoughts.
Regards,
Boqun
--------------->8
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb68f27..938a5053def3 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -84,14 +84,18 @@ static inline void __seqcount_init(seqcount_t *s, const char *name,
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname }
+# define MSIOCU 8 /* MAX SEQCOUNT IN ON COMPILE UNIT */
/**
* seqcount_init() - runtime initializer for seqcount_t
* @s: Pointer to the seqcount_t instance
*/
# define seqcount_init(s) \
do { \
- static struct lock_class_key __key; \
- __seqcount_init((s), #s, &__key); \
+ static struct lock_class_key __key[MSIOCU]; \
+ static int idx = 0; \
+ \
+ BUG_ON(idx >= MSIOCU); \
+ __seqcount_init((s), #s, &__key[idx++]); \
} while (0)
static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-15 12:48 ` Boqun Feng
2020-09-15 13:10 ` Boqun Feng
@ 2020-09-15 14:30 ` peterz
2020-09-16 12:52 ` Qian Cai
1 sibling, 1 reply; 9+ messages in thread
From: peterz @ 2020-09-15 14:30 UTC (permalink / raw)
To: Boqun Feng
Cc: Qian Cai, Ahmed S. Darwish, Ingo Molnar, Will Deacon,
Thomas Gleixner, Sebastian A. Siewior, Paul E. McKenney,
Steven Rostedt, LKML, Stephen Rothwell, linux-next, Waiman Long
On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> I think this happened because seqcount_##lockname##_init() is defined at
> function rather than macro, so when the seqcount_init() gets expand in
Bah! I hate all this :/
I suspect the below, while more verbose than I'd like is the best
option.
---
include/linux/seqlock.h | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f73c7eb68f27..76e44e6c0100 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* @lock: Pointer to the associated lock
*/
+#define seqcount_LOCKNAME_init(s, _lock, lockname) \
+ do { \
+ seqcount_##lockname##_t *____s = (s); \
+ seqcount_init(&____s->seqcount); \
+ __SEQ_LOCK(____s->lock = (_lock)); \
+ } while (0)
+
+#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, raw_spinlock)
+#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, spinlock)
+#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s, lock, rwlock);
+#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, mutex);
+#define seqcount_ww_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock, ww_mutex);
+
/*
* SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
* seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
@@ -190,13 +203,6 @@ typedef struct seqcount_##lockname { \
__SEQ_LOCK(locktype *lock); \
} seqcount_##lockname##_t; \
\
-static __always_inline void \
-seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock) \
-{ \
- seqcount_init(&s->seqcount); \
- __SEQ_LOCK(s->lock = lock); \
-} \
- \
static __always_inline seqcount_t * \
__seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
{ \
@@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mu
__SEQ_LOCK(.lock = (assoc_lock)) \
}
-#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
#define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
+#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
#define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
#define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
#define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name, lock)
^ permalink raw reply related [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-15 14:30 ` peterz
@ 2020-09-16 12:52 ` Qian Cai
2020-09-16 12:54 ` peterz
0 siblings, 1 reply; 9+ messages in thread
From: Qian Cai @ 2020-09-16 12:52 UTC (permalink / raw)
To: peterz, Boqun Feng, Stephen Rothwell
Cc: Ahmed S. Darwish, Ingo Molnar, Will Deacon, Thomas Gleixner,
Sebastian A. Siewior, Paul E. McKenney, Steven Rostedt, LKML,
linux-next, Waiman Long
On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > I think this happened because seqcount_##lockname##_init() is defined at
> > function rather than macro, so when the seqcount_init() gets expand in
>
> Bah! I hate all this :/
>
> I suspect the below, while more verbose than I'd like is the best
> option.
Stephen, can you add this patch for now until Peter beats you to it?
>
> ---
> include/linux/seqlock.h | 22 ++++++++++++++--------
> 1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index f73c7eb68f27..76e44e6c0100 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -173,6 +173,19 @@ static inline void seqcount_lockdep_reader_access(const
> seqcount_t *s)
> * @lock: Pointer to the associated lock
> */
>
> +#define seqcount_LOCKNAME_init(s, _lock, lockname) \
> + do { \
> + seqcount_##lockname##_t *____s = (s); \
> + seqcount_init(&____s->seqcount); \
> + __SEQ_LOCK(____s->lock = (_lock)); \
> + } while (0)
> +
> +#define seqcount_raw_spinlock_init(s, lock) seqcount_LOCKNAME_init(s, lock,
> raw_spinlock)
> +#define seqcount_spinlock_init(s, lock) seqcount_LOCKNAME_init(s
> , lock, spinlock)
> +#define seqcount_rwlock_init(s, lock) seqcount_LOCKNAME_init(s
> , lock, rwlock);
> +#define seqcount_mutex_init(s, lock) seqcount_LOCKNAME_init(s, lock,
> mutex);
> +#define seqcount_ww_mutex_init(s, lock) seqcount_LOCKNAME_init(s
> , lock, ww_mutex);
> +
> /*
> * SEQCOUNT_LOCKNAME() - Instantiate seqcount_LOCKNAME_t and helpers
> * seqprop_LOCKNAME_*() - Property accessors for seqcount_LOCKNAME_t
> @@ -190,13 +203,6 @@ typedef struct seqcount_##lockname {
> \
> __SEQ_LOCK(locktype *lock); \
> } seqcount_##lockname##_t; \
> \
> -static __always_inline void \
> -seqcount_##lockname##_init(seqcount_##lockname##_t *s, locktype *lock)
> \
> -{ \
> - seqcount_init(&s->seqcount); \
> - __SEQ_LOCK(s->lock = lock); \
> -} \
> - \
> static __always_inline seqcount_t * \
> __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s)
> \
> { \
> @@ -284,8 +290,8 @@ SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex,
> true, &s->lock->base, ww_mu
> __SEQ_LOCK(.lock = (assoc_lock)) \
> }
>
> -#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> #define SEQCNT_RAW_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> +#define SEQCNT_SPINLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
> #define SEQCNT_RWLOCK_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(n
> ame, lock)
> #define SEQCNT_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(n
> ame, lock)
> #define SEQCNT_WW_MUTEX_ZERO(name, lock) SEQCOUNT_LOCKNAME_ZERO(name,
> lock)
>
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-16 12:52 ` Qian Cai
@ 2020-09-16 12:54 ` peterz
2020-09-16 13:00 ` Qian Cai
0 siblings, 1 reply; 9+ messages in thread
From: peterz @ 2020-09-16 12:54 UTC (permalink / raw)
To: Qian Cai
Cc: Boqun Feng, Stephen Rothwell, Ahmed S. Darwish, Ingo Molnar,
Will Deacon, Thomas Gleixner, Sebastian A. Siewior,
Paul E. McKenney, Steven Rostedt, LKML, linux-next, Waiman Long
On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > I think this happened because seqcount_##lockname##_init() is defined at
> > > function rather than macro, so when the seqcount_init() gets expand in
> >
> > Bah! I hate all this :/
> >
> > I suspect the below, while more verbose than I'd like is the best
> > option.
>
> Stephen, can you add this patch for now until Peter beats you to it?
Did you verify it works? I only wrote it..
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-16 12:54 ` peterz
@ 2020-09-16 13:00 ` Qian Cai
2020-09-16 13:02 ` peterz
0 siblings, 1 reply; 9+ messages in thread
From: Qian Cai @ 2020-09-16 13:00 UTC (permalink / raw)
To: peterz
Cc: Qian Cai, Boqun Feng, Stephen Rothwell, Ahmed S. Darwish,
Ingo Molnar, Will Deacon, Thomas Gleixner, Sebastian A. Siewior,
Paul E. McKenney, Steven Rostedt, LKML, linux-next, Waiman Long
----- Original Message -----
> On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> > On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > > I think this happened because seqcount_##lockname##_init() is defined
> > > > at
> > > > function rather than macro, so when the seqcount_init() gets expand in
> > >
> > > Bah! I hate all this :/
> > >
> > > I suspect the below, while more verbose than I'd like is the best
> > > option.
> >
> > Stephen, can you add this patch for now until Peter beats you to it?
>
> Did you verify it works? I only wrote it..
Yes, I did.
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-16 13:00 ` Qian Cai
@ 2020-09-16 13:02 ` peterz
2020-09-17 2:31 ` Stephen Rothwell
0 siblings, 1 reply; 9+ messages in thread
From: peterz @ 2020-09-16 13:02 UTC (permalink / raw)
To: Qian Cai
Cc: Qian Cai, Boqun Feng, Stephen Rothwell, Ahmed S. Darwish,
Ingo Molnar, Will Deacon, Thomas Gleixner, Sebastian A. Siewior,
Paul E. McKenney, Steven Rostedt, LKML, linux-next, Waiman Long
On Wed, Sep 16, 2020 at 09:00:59AM -0400, Qian Cai wrote:
>
>
> ----- Original Message -----
> > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> > > On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > > > I think this happened because seqcount_##lockname##_init() is defined
> > > > > at
> > > > > function rather than macro, so when the seqcount_init() gets expand in
> > > >
> > > > Bah! I hate all this :/
> > > >
> > > > I suspect the below, while more verbose than I'd like is the best
> > > > option.
> > >
> > > Stephen, can you add this patch for now until Peter beats you to it?
> >
> > Did you verify it works? I only wrote it..
>
> Yes, I did.
Excellent, I'll stick a Tested-by from you on then.
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support
2020-09-16 13:02 ` peterz
@ 2020-09-17 2:31 ` Stephen Rothwell
0 siblings, 0 replies; 9+ messages in thread
From: Stephen Rothwell @ 2020-09-17 2:31 UTC (permalink / raw)
To: peterz
Cc: Qian Cai, Qian Cai, Boqun Feng, Ahmed S. Darwish, Ingo Molnar,
Will Deacon, Thomas Gleixner, Sebastian A. Siewior,
Paul E. McKenney, Steven Rostedt, LKML, linux-next, Waiman Long
[-- Attachment #1: Type: text/plain, Size: 1110 bytes --]
Hi all,
On Wed, 16 Sep 2020 15:02:33 +0200 peterz@infradead.org wrote:
>
> On Wed, Sep 16, 2020 at 09:00:59AM -0400, Qian Cai wrote:
> >
> >
> > ----- Original Message -----
> > > On Wed, Sep 16, 2020 at 08:52:07AM -0400, Qian Cai wrote:
> > > > On Tue, 2020-09-15 at 16:30 +0200, peterz@infradead.org wrote:
> > > > > On Tue, Sep 15, 2020 at 08:48:17PM +0800, Boqun Feng wrote:
> > > > > > I think this happened because seqcount_##lockname##_init() is defined
> > > > > > at
> > > > > > function rather than macro, so when the seqcount_init() gets expand in
> > > > >
> > > > > Bah! I hate all this :/
> > > > >
> > > > > I suspect the below, while more verbose than I'd like is the best
> > > > > option.
> > > >
> > > > Stephen, can you add this patch for now until Peter beats you to it?
> > >
> > > Did you verify it works? I only wrote it..
> >
> > Yes, I did.
>
> Excellent, I'll stick a Tested-by from you on then.
I'll add this into the tip tree merge today (unless the tip tree is
updated in the mean time).
--
Cheers,
Stephen Rothwell
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]
^ permalink raw reply [flat|nested] 9+ messages in thread
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Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
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2020-09-15 0:20 ` [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support Qian Cai
2020-09-15 12:48 ` Boqun Feng
2020-09-15 13:10 ` Boqun Feng
2020-09-15 14:30 ` peterz
2020-09-16 12:52 ` Qian Cai
2020-09-16 12:54 ` peterz
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2020-09-17 2:31 ` Stephen Rothwell
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