Re: [PATCH] rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock()

From: Joel Fernandes
Date: Fri Jan 12 2024 - 08:51:14 EST

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On 1/12/2024 2:24 AM, Z qiang wrote:
>>
>>
>>
>> On 1/11/2024 6:54 AM, Paul E. McKenney wrote:
>>> On Wed, Jan 10, 2024 at 04:36:46PM +0800, Z qiang wrote:
>>>>>
>>>>> For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and
>>>>> CONFIG_RCU_LAZY=y, here are the following scenarios that will trigger
>>>>> WARN_ON_ONCE() in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended().
>>>>>
>>>>> CPU2 CPU11
>>>>> kthread
>>>>> rcu_nocb_cb_kthread ksys_write
>>>>> rcu_do_batch vfs_write
>>>>> rcu_torture_timer_cb proc_sys_write
>>>>> __kmem_cache_free proc_sys_call_handler
>>>>> kmemleak_free drop_caches_sysctl_handler
>>>>> delete_object_full drop_slab
>>>>> __delete_object shrink_slab
>>>>> put_object lazy_rcu_shrink_scan
>>>>> call_rcu rcu_nocb_flush_bypass
>>>>> __call_rcu_commn rcu_nocb_bypass_lock
>>>>> raw_spin_trylock(&rdp->nocb_bypass_lock) fail
>>>>> atomic_inc(&rdp->nocb_lock_contended);
>>>>> rcu_nocb_wait_contended WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
>>>>> WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended)) |
>>>>> |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __|
>>>>>
>>>>> This commit therefore use the rcu_nocb_try_flush_bypass() instead of
>>>>> rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan(), if the nocb_bypass
>>>>> queue is being flushed, the rcu_nocb_try_flush_bypass will return directly.
>>>>>
>>>>> Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
>>>
>>> Just to make sure I understand, the "echo" command called out below
>>> will trigger the two-CPU scenario called out above in kernels built with
>>> CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and CONFIG_RCU_LAZY=y, correct?
>>>
>>> If so, good catch!
>>>
>>> Any objections to this patch? Or to put it another way, is there a
>>> better fix via adjusting lazy RCU?
>>
>> I think it is a good find and no objections to this patch. One thing we could
>> also do is have rcu_nocb_try_flush_bypass() return false if the
>> trylock fails, and then retry till lock is available. That would give us
>> roughly the same behavior as rcu_nocb_flush_bypass() but I am not sure if it is
>> worth it, because the shrinker will just try again if memory pressure is not
>> relieved anyway.
>>
>> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> Thanks Joel for the review :) .

You're welcome! And thanks for the patch. :)

- Joel

>
>>
>> thanks,
>>
>> - Joel
>>
>>
>>>
>>> Thanx, Paul
>>>
>>>>> ---
>>>>
>>>> During rcutorture testing, use echo 3 > /proc/sys/vm/drop_caches will trigger:
>>>>
>>>> [ 52.674359] WARNING: CPU: 11 PID: 505 at kernel/rcu/tree_nocb.h:104
>>>> rcu_nocb_bypass_lock+0xc7/0xd0
>>>> [ 52.674388] Modules linked in:
>>>> [ 52.674406] CPU: 11 PID: 505 Comm: sh Not tainted 6.6.0-rt14zqiang-dirty #103
>>>> [ 52.674422] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
>>>> rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
>>>> [ 52.674436] RIP: 0010:rcu_nocb_bypass_lock+0xc7/0xd0
>>>> [ 52.674454] Code: 4c 89 e7 e8 4b 3d 6a 01 be 04 00 00 00 4c 89 ef e8
>>>> ce 92 31 00 f0 ff 8b 68 02 00 00 5b 41 5c 41 5d 41 5e 5d c3 cc cc cc
>>>> cc 90 <0f> 0b 90 eb d1 0f 1f 40 00 90 90 0
>>>> [ 52.674467] RSP: 0018:ffff88800af6fa68 EFLAGS: 00010093
>>>> [ 52.674487] RAX: 0000000000000000 RBX: ffff888069e0f540 RCX: ffffffffb5c12d44
>>>> [ 52.674497] RDX: 0000000000000003 RSI: dffffc0000000000 RDI: ffff888069e0fb10
>>>> [ 52.674508] RBP: ffff88800af6fa88 R08: ffffed100d3c1ef6 R09: ffffed100d3c1ef6
>>>> [ 52.674518] R10: ffffed100d3c1ef5 R11: ffff888069e0f7ab R12: ffff888069e0f8c0
>>>> [ 52.674529] R13: ffff888069e0f7a8 R14: 000000000000000b R15: ffff88800af6fb90
>>>> [ 52.674540] FS: 00007ff543132740(0000) GS:ffff88806c000000(0000)
>>>> knlGS:0000000000000000
>>>> [ 52.674555] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [ 52.674565] CR2: 000055df13465004 CR3: 00000000027f2000 CR4: 00000000001506e0
>>>> [ 52.674576] Call Trace:
>>>> [ 52.674583] <TASK>
>>>> [ 52.674598] ? show_regs+0x66/0x70
>>>> [ 52.674627] ? __warn+0xae/0x220
>>>> [ 52.674657] ? rcu_nocb_bypass_lock+0xc7/0xd0
>>>> [ 52.674693] ? report_bug+0x14a/0x240
>>>> [ 52.674756] ------------[ cut here ]------------
>>>> [ 52.674766] WARNING: CPU: 2 PID: 118 at kernel/rcu/tree_nocb.h:124
>>>> __call_rcu_common+0xd3f/0xd80
>>>> [ 52.674785] Modules linked in:
>>>> [ 52.674785] ? handle_bug+0x44/0x80
>>>> [ 52.674795] CPU: 2 PID: 118 Comm: rcuop/10 Not tainted
>>>> 6.6.0-rt14zqiang-dirty #103
>>>> [ 52.674806] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
>>>> rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
>>>> [ 52.674812] RIP: 0010:__call_rcu_common+0xd3f/0xd80
>>>> [ 52.674816] ? exc_invalid_op+0x1c/0x50
>>>> [ 52.674822] Code: 9e 02 4c 89 e2 e8 a1 46 ff ff e9 d0 fa ff ff 48 8d
>>>> 7b 18 e8 43 5f 30 00 48 8b 7b 18 48 89 de e8 67 9a ff ff e9 69 f6 ff
>>>> ff 90 <0f> 0b 90 f3 90 e9 17 fa ff ff 90 b
>>>> [ 52.674832] RSP: 0018:ffff888002277af8 EFLAGS: 00010002
>>>> [ 52.674847] RAX: 0000000000000001 RBX: ffff888069e0f540 RCX: ffffffffb5c24655
>>>> [ 52.674848] ? asm_exc_invalid_op+0x1f/0x30
>>>> [ 52.674857] RDX: 0000000000000003 RSI: dffffc0000000000 RDI: ffff888069e0f7a8
>>>> [ 52.674867] RBP: ffff888002277bf0 R08: ffffed100d3c1ef6 R09: ffffed100d3c1ef6
>>>> [ 52.674877] R10: ffffed100d3c1ef5 R11: ffff888069e0f7ab R12: ffff88800bab62f8
>>>> [ 52.674887] R13: ffff888069e0f7a8 R14: 0000000000000000 R15: ffff888069e0f918
>>>> [ 52.674897] FS: 0000000000000000(0000) GS:ffff888069c00000(0000)
>>>> knlGS:0000000000000000
>>>> [ 52.674910] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [ 52.674920] CR2: 000055df13419911 CR3: 00000000027f2000 CR4: 00000000001506e0
>>>> [ 52.674930] Call Trace:
>>>> [ 52.674936] <TASK>
>>>> [ 52.674934] ? rcu_nocb_bypass_lock+0x94/0xd0
>>>> [ 52.674950] ? show_regs+0x66/0x70
>>>> [ 52.674972] ? rcu_nocb_bypass_lock+0xc7/0xd0
>>>> [ 52.674974] ? __warn+0xae/0x220
>>>> [ 52.675002] ? __call_rcu_common+0xd3f/0xd80
>>>> [ 52.675010] ? rcu_nocb_bypass_lock+0x94/0xd0
>>>> [ 52.675037] ? report_bug+0x14a/0x240
>>>> [ 52.675054] rcu_nocb_flush_bypass+0x3a/0x60
>>>> [ 52.675094] ? handle_bug+0x44/0x80
>>>> [ 52.675095] lazy_rcu_shrink_scan+0x12e/0x220
>>>> [ 52.675114] ? exc_invalid_op+0x1c/0x50
>>>> [ 52.675139] ? asm_exc_invalid_op+0x1f/0x30
>>>> [ 52.675156] shrink_slab.constprop.116+0x2cd/0x6e0
>>>> [ 52.675206] ? __call_rcu_common+0x775/0xd80
>>>> [ 52.675239] ? __call_rcu_common+0xd3f/0xd80
>>>> [ 52.675259] ? __pfx_shrink_slab.constprop.116+0x10/0x10
>>>> [ 52.675272] ? __call_rcu_common+0x775/0xd80
>>>> [ 52.675335] ? __pfx_drop_pagecache_sb+0x10/0x10
>>>> [ 52.675365] ? __pfx___call_rcu_common+0x10/0x10
>>>> [ 52.675386] ? preempt_schedule+0x7f/0xa0
>>>> [ 52.675396] drop_slab+0x64/0x90
>>>> [ 52.675413] ? preempt_schedule_thunk+0x1a/0x30
>>>> [ 52.675426] drop_caches_sysctl_handler+0x82/0xe0
>>>> [ 52.675472] call_rcu+0x17/0x20
>>>> [ 52.675489] put_object+0x53/0x70
>>>> [ 52.675513] __delete_object+0x73/0x90
>>>> [ 52.675545] delete_object_full+0x1f/0x30
>>>> [ 52.675563] kmemleak_free+0x41/0x70
>>>> [ 52.675586] __kmem_cache_free+0x1bd/0x230
>>>> [ 52.675598] ? rcu_torture_timer_cb+0x12/0x20
>>>> [ 52.675623] ? rcu_do_batch+0x466/0xf50
>>>> [ 52.675649] kfree+0x90/0x110
>>>> [ 52.675661] ? __pfx_rcu_torture_timer_cb+0x10/0x10
>>>> [ 52.675678] rcu_torture_timer_cb+0x12/0x20
>>>> [ 52.675697] rcu_do_batch+0x46b/0xf50
>>>> [ 52.675464] proc_sys_call_handler+0x247/0x310
>>>> [ 52.675772] ? __pfx_rcu_do_batch+0x10/0x10
>>>> [ 52.675785] ? migrate_disable+0x2a/0xf0
>>>> [ 52.675815] ? lockdep_softirqs_off+0x13d/0x200
>>>> [ 52.675854] ? rcu_nocb_cb_kthread+0x29c/0x880
>>>> [ 52.675889] rcu_nocb_cb_kthread+0x2b1/0x880
>>>> [ 52.675910] ? __pfx_proc_sys_call_handler+0x10/0x10
>>>> [ 52.675945] ? vfs_write+0x3ea/0x7c0
>>>> [ 52.675958] ? vfs_write+0x3ea/0x7c0
>>>> [ 52.675959] ? __pfx_rcu_nocb_cb_kthread+0x10/0x10
>>>> [ 52.675974] ? trace_preempt_on+0x54/0xe0
>>>> [ 52.675990] ? __kthread_parkme+0x80/0x110
>>>> [ 52.676015] ? preempt_count_sub+0x50/0x80
>>>> [ 52.676031] proc_sys_write+0x17/0x20
>>>> [ 52.676050] vfs_write+0x58b/0x7c0
>>>> [ 52.676063] ? __kthread_parkme+0xf2/0x110
>>>> [ 52.676111] ? __pfx_rcu_nocb_cb_kthread+0x10/0x10
>>>> [ 52.676112] ? __pfx_vfs_write+0x10/0x10
>>>> [ 52.676139] kthread+0x1a8/0x1f0
>>>> [ 52.676161] ? kthread+0x107/0x1f0
>>>> [ 52.676163] ? __might_fault+0x84/0xd0
>>>> [ 52.676183] ? __pfx_kthread+0x10/0x10
>>>> [ 52.676197] ? __might_fault+0xbe/0xd0
>>>> [ 52.676213] ? __might_fault+0x84/0xd0
>>>> [ 52.676223] ret_from_fork+0x40/0x60
>>>> [ 52.676238] ? __pfx_kthread+0x10/0x10
>>>> [ 52.676272] ? __fget_light+0xb8/0x120
>>>> [ 52.676273] ret_from_fork_asm+0x1b/0x30
>>>> [ 52.676355] ksys_write+0xd0/0x170
>>>> [ 52.676386] ? __pfx_ksys_write+0x10/0x10
>>>> [ 52.676418] </TASK>
>>>> [ 52.676425] irq event stamp: 591689
>>>> [ 52.676433] hardirqs last enabled at (591688): [<ffffffffb72b7193>]
>>>> _raw_spin_unlock_irqrestore+0x63/0x80
>>>> [ 52.676452] hardirqs last disabled at (591689): [<ffffffffb5c242f3>]
>>>> __call_rcu_common+0x413/0xd80
>>>> [ 52.676467] softirqs last enabled at (591668): [<ffffffffb5af3489>]
>>>> __local_bh_enable_ip+0x109/0x160
>>>> [ 52.676486] softirqs last disabled at (591672): [<ffffffffb5c1ed2d>]
>>>> rcu_do_batch+0x5ad/0xf50
>>>> [ 52.676488] __x64_sys_write+0x47/0x60
>>>> [ 52.676500] ---[ end trace 0000000000000000 ]---
>>>> root@qemux86-64:~# [ 52.676526] do_syscall_64+0x47/0x90
>>>> [ 52.676552] entry_SYSCALL_64_after_hwframe+0x6f/0xd9
>>>> [ 52.676566] RIP: 0033:0x7ff5432260c4
>>>> [ 52.676583] Code: 15 59 7d 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff
>>>> eb b7 0f 1f 00 f3 0f 1e fa 80 3d 1d 0d 0e 00 00 74 13 b8 01 00 00 00
>>>> 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 8
>>>> [ 52.676596] RSP: 002b:00007ffcc9614998 EFLAGS: 00000202 ORIG_RAX:
>>>> 0000000000000001
>>>> [ 52.676613] RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007ff5432260c4
>>>> [ 52.676624] RDX: 0000000000000002 RSI: 000055df1345df40 RDI: 0000000000000001
>>>> [ 52.676634] RBP: 000055df1345df40 R08: 000055df1345b700 R09: 0000000000000000
>>>> [ 52.676644] R10: 00000000000001b6 R11: 0000000000000202 R12: 0000000000000001
>>>> [ 52.676653] R13: 00007ff5431326c8 R14: 00007ffcc9614a00 R15: 000055df1345b6f8
>>>> [ 52.676789] </TASK>
>>>> [ 52.676796] irq event stamp: 502888
>>>> [ 52.676804] hardirqs last enabled at (502887): [<ffffffffb72b7193>]
>>>> _raw_spin_unlock_irqrestore+0x63/0x80
>>>> [ 52.676823] hardirqs last disabled at (502888): [<ffffffffb5c19753>]
>>>> lazy_rcu_shrink_scan+0x1d3/0x220
>>>> [ 52.676838] softirqs last enabled at (54708): [<ffffffffb5af3489>]
>>>> __local_bh_enable_ip+0x109/0x160
>>>> [ 52.676854] softirqs last disabled at (54698): [<ffffffffb6fd861a>]
>>>> unix_release_sock+0x26a/0x7c0
>>>> [ 52.676888] ---[ end trace 0000000000000000 ]---
>>>>
>>>> Thanks
>>>> Zqiang
>>>>
>>>>
>>>>> kernel/rcu/tree_nocb.h | 2 +-
>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
>>>>> index d82f96a66600..9b618842c324 100644
>>>>> --- a/kernel/rcu/tree_nocb.h
>>>>> +++ b/kernel/rcu/tree_nocb.h
>>>>> @@ -1381,7 +1381,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>>>>> rcu_nocb_unlock_irqrestore(rdp, flags);
>>>>> continue;
>>>>> }
>>>>> - WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
>>>>> + rcu_nocb_try_flush_bypass(rdp, jiffies);
>>>>> rcu_nocb_unlock_irqrestore(rdp, flags);
>>>>> wake_nocb_gp(rdp, false);
>>>>> sc->nr_to_scan -= _count;
>>>>> --
>>>>> 2.17.1
>>>>>
>>>

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