Re: [PATCH v2 5.12] io_uring: don't hold uring_lock when calling io_run_task_work*

[Jens Axboe <axboe@kernel.dk>]

On 2/18/21 7:39 AM, Hao Xu wrote:
> Abaci reported the below issue:
> [  141.400455] hrtimer: interrupt took 205853 ns
> [  189.869316] process 'usr/local/ilogtail/ilogtail_0.16.26' started with executable stack
> [  250.188042]
> [  250.188327] ============================================
> [  250.189015] WARNING: possible recursive locking detected
> [  250.189732] 5.11.0-rc4 #1 Not tainted
> [  250.190267] --------------------------------------------
> [  250.190917] a.out/7363 is trying to acquire lock:
> [  250.191506] ffff888114dbcbe8 (&ctx->uring_lock){+.+.}-{3:3}, at: __io_req_task_submit+0x29/0xa0
> [  250.192599]
> [  250.192599] but task is already holding lock:
> [  250.193309] ffff888114dbfbe8 (&ctx->uring_lock){+.+.}-{3:3}, at: __x64_sys_io_uring_register+0xad/0x210
> [  250.194426]
> [  250.194426] other info that might help us debug this:
> [  250.195238]  Possible unsafe locking scenario:
> [  250.195238]
> [  250.196019]        CPU0
> [  250.196411]        ----
> [  250.196803]   lock(&ctx->uring_lock);
> [  250.197420]   lock(&ctx->uring_lock);
> [  250.197966]
> [  250.197966]  *** DEADLOCK ***
> [  250.197966]
> [  250.198837]  May be due to missing lock nesting notation
> [  250.198837]
> [  250.199780] 1 lock held by a.out/7363:
> [  250.200373]  #0: ffff888114dbfbe8 (&ctx->uring_lock){+.+.}-{3:3}, at: __x64_sys_io_uring_register+0xad/0x210
> [  250.201645]
> [  250.201645] stack backtrace:
> [  250.202298] CPU: 0 PID: 7363 Comm: a.out Not tainted 5.11.0-rc4 #1
> [  250.203144] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
> [  250.203887] Call Trace:
> [  250.204302]  dump_stack+0xac/0xe3
> [  250.204804]  __lock_acquire+0xab6/0x13a0
> [  250.205392]  lock_acquire+0x2c3/0x390
> [  250.205928]  ? __io_req_task_submit+0x29/0xa0
> [  250.206541]  __mutex_lock+0xae/0x9f0
> [  250.207071]  ? __io_req_task_submit+0x29/0xa0
> [  250.207745]  ? 0xffffffffa0006083
> [  250.208248]  ? __io_req_task_submit+0x29/0xa0
> [  250.208845]  ? __io_req_task_submit+0x29/0xa0
> [  250.209452]  ? __io_req_task_submit+0x5/0xa0
> [  250.210083]  __io_req_task_submit+0x29/0xa0
> [  250.210687]  io_async_task_func+0x23d/0x4c0
> [  250.211278]  task_work_run+0x89/0xd0
> [  250.211884]  io_run_task_work_sig+0x50/0xc0
> [  250.212464]  io_sqe_files_unregister+0xb2/0x1f0
> [  250.213109]  __io_uring_register+0x115a/0x1750
> [  250.213718]  ? __x64_sys_io_uring_register+0xad/0x210
> [  250.214395]  ? __fget_files+0x15a/0x260
> [  250.214956]  __x64_sys_io_uring_register+0xbe/0x210
> [  250.215620]  ? trace_hardirqs_on+0x46/0x110
> [  250.216205]  do_syscall_64+0x2d/0x40
> [  250.216731]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [  250.217455] RIP: 0033:0x7f0fa17e5239
> [  250.218034] Code: 01 00 48 81 c4 80 00 00 00 e9 f1 fe ff ff 0f 1f 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05  3d 01 f0 ff ff 73 01 c3 48 8b 0d 27 ec 2c 00 f7 d8 64 89 01 48
> [  250.220343] RSP: 002b:00007f0fa1eeac48 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
> [  250.221360] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f0fa17e5239
> [  250.222272] RDX: 0000000000000000 RSI: 0000000000000003 RDI: 0000000000000008
> [  250.223185] RBP: 00007f0fa1eeae20 R08: 0000000000000000 R09: 0000000000000000
> [  250.224091] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> [  250.224999] R13: 0000000000021000 R14: 0000000000000000 R15: 00007f0fa1eeb700
> 
> This is caused by calling io_run_task_work_sig() to do work under
> uring_lock while the caller io_sqe_files_unregister() already held
> uring_lock.
> To fix this issue, briefly drop uring_lock when calling
> io_run_task_work_sig(), and there are two things to concern:
>   - hold uring_lock in io_ring_ctx_free() when calling io_sqe_files_unregister()
>       this is for consistency of lock/unlock.
>   - add new fixed file ref node before dropping uring_lock
>       it's not safe to do io_uring_enter-->percpu_ref_get() with a dying one.
>   - check if file_data->refs is dying to avoid parallel io_sqe_files_unregister

This no longer applies to for-5.12/io_uring - care to recheck and respin?

-- 
Jens Axboe