Re: [RFC v7 12/19] lockdep: Add recursive read locks into dependency graph

From: Qian Cai
Date: Mon Sep 14 2020 - 14:17:14 EST

On Fri, 2020-08-07 at 15:42 +0800, Boqun Feng wrote:
> Since we have all the fundamental to handle recursive read locks, we now
> add them into the dependency graph.
>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>

Reverting this patch and its dependency:

[14/19] lockdep: Take read/write status in consideration when generate chainkey

fixed a splat below. IOW, this patch introduced this new splat which looks like
a false positive because the existing locking dependency chains 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 seqcount in
read_mems_allowed_begin() is read_seqcount_begin(&current->mems_allowed_seq), so
there should be no deadlock?

[ 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

> ---
> kernel/locking/lockdep.c | 19 ++-----------------
> 1 file changed, 2 insertions(+), 17 deletions(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 040509667798..867199c4b85d 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2808,16 +2808,6 @@ check_prev_add(struct task_struct *curr, struct
> held_lock *prev,
> if (!check_irq_usage(curr, prev, next))
> return 0;
>
> - /*
> - * For recursive read-locks we do all the dependency checks,
> - * but we dont store read-triggered dependencies (only
> - * write-triggered dependencies). This ensures that only the
> - * write-side dependencies matter, and that if for example a
> - * write-lock never takes any other locks, then the reads are
> - * equivalent to a NOP.
> - */
> - if (next->read == 2 || prev->read == 2)
> - return 1;
> /*
> * Is the <prev> -> <next> dependency already present?
> *
> @@ -2935,13 +2925,8 @@ check_prevs_add(struct task_struct *curr, struct
> held_lock *next)
> u16 distance = curr->lockdep_depth - depth + 1;
> hlock = curr->held_locks + depth - 1;
>
> - /*
> - * Only non-recursive-read entries get new dependencies
> - * added:
> - */
> - if (hlock->read != 2 && hlock->check) {
> - int ret = check_prev_add(curr, hlock, next, distance,
> - &trace);
> + if (hlock->check) {
> + int ret = check_prev_add(curr, hlock, next, distance,
> &trace);
> if (!ret)
> return 0;
>