Subject: Re: [PATCH v2 0/5] seqlock: Introduce PREEMPT_RT support

From: Qian Cai <[email-masked]>
Date: Mon, 14 Sep 2020 20:20:53 -0400

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/[email-masked]
> 
> 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(&current->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