[PATCH 2/2] io_uring: don't hold uring_lock when calling io_run_task_work*

Hao Xu
2021-02-03T15:24:57Z

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.
we need to check if uring_lock is held by us when doing unlock around
io_run_task_work_sig() since there are code paths down to that place
without uring_lock held.

Reported-by: Abaci <abaci@linux.alibaba.com>
Fixes: 1ffc54220c44 ("io_uring: fix io_sqe_files_unregister() hangs")
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
---
fs/io_uring.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index efb6d02fea6f..b093977713ee 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7362,18 +7362,25 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx, bool locked)

/* wait for all refs nodes to complete */
flush_delayed_work(&ctx->file_put_work);
+ if (locked)
+ mutex_unlock(&ctx->uring_lock);
do {
ret = wait_for_completion_interruptible(&data->done);
if (!ret)
break;
ret = io_run_task_work_sig();
- if (ret < 0) {
- percpu_ref_resurrect(&data->refs);
- reinit_completion(&data->done);
- io_sqe_files_set_node(data, backup_node);
- return ret;
- }
+ if (ret < 0)
+ break;
} while (1);
+ if (locked)
+ mutex_lock(&ctx->uring_lock);
+
+ if (ret < 0) {
+ percpu_ref_resurrect(&data->refs);
+ reinit_completion(&data->done);
+ io_sqe_files_set_node(data, backup_node);
+ return ret;
+ }

__io_sqe_files_unregister(ctx);
nr_tables = DIV_ROUND_UP(ctx->nr_user_files, IORING_MAX_FILES_TABLE);
--
1.8.3.1