[PATCH 5.13 018/151] io_uring: drop ctx->uring_lock before flushing work item

Greg Kroah-Hartman
2021-08-16T13:19:56Z

From: Jens Axboe <axboe@kernel.dk>

commit c018db4a57f3e31a9cb24d528e9f094eda89a499 upstream.

Ammar reports that he's seeing a lockdep splat on running test/rsrc_tags
from the regression suite:

======================================================
WARNING: possible circular locking dependency detected
5.14.0-rc3-bluetea-test-00249-gc7d102232649 #5 Tainted: G OE
------------------------------------------------------
kworker/2:4/2684 is trying to acquire lock:
ffff88814bb1c0a8 (&ctx->uring_lock){+.+.}-{3:3}, at: io_rsrc_put_work+0x13d/0x1a0

but task is already holding lock:
ffffc90001c6be70 ((work_completion)(&(&ctx->rsrc_put_work)->work)){+.+.}-{0:0}, at: process_one_work+0x1bc/0x530

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 ((work_completion)(&(&ctx->rsrc_put_work)->work)){+.+.}-{0:0}:
__flush_work+0x31b/0x490
io_rsrc_ref_quiesce.part.0.constprop.0+0x35/0xb0
__do_sys_io_uring_register+0x45b/0x1060
do_syscall_64+0x35/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #0 (&ctx->uring_lock){+.+.}-{3:3}:
__lock_acquire+0x119a/0x1e10
lock_acquire+0xc8/0x2f0
__mutex_lock+0x86/0x740
io_rsrc_put_work+0x13d/0x1a0
process_one_work+0x236/0x530
worker_thread+0x52/0x3b0
kthread+0x135/0x160
ret_from_fork+0x1f/0x30

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock((work_completion)(&(&ctx->rsrc_put_work)->work));
lock(&ctx->uring_lock);
lock((work_completion)(&(&ctx->rsrc_put_work)->work));
lock(&ctx->uring_lock);

*** DEADLOCK ***

2 locks held by kworker/2:4/2684:
#0: ffff88810004d938 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x1bc/0x530
#1: ffffc90001c6be70 ((work_completion)(&(&ctx->rsrc_put_work)->work)){+.+.}-{0:0}, at: process_one_work+0x1bc/0x530

stack backtrace:
CPU: 2 PID: 2684 Comm: kworker/2:4 Tainted: G OE 5.14.0-rc3-bluetea-test-00249-gc7d102232649 #5
Hardware name: Acer Aspire ES1-421/OLVIA_BE, BIOS V1.05 07/02/2015
Workqueue: events io_rsrc_put_work
Call Trace:
dump_stack_lvl+0x6a/0x9a
check_noncircular+0xfe/0x110
__lock_acquire+0x119a/0x1e10
lock_acquire+0xc8/0x2f0
? io_rsrc_put_work+0x13d/0x1a0
__mutex_lock+0x86/0x740
? io_rsrc_put_work+0x13d/0x1a0
? io_rsrc_put_work+0x13d/0x1a0
? io_rsrc_put_work+0x13d/0x1a0
? process_one_work+0x1ce/0x530
io_rsrc_put_work+0x13d/0x1a0
process_one_work+0x236/0x530
worker_thread+0x52/0x3b0
? process_one_work+0x530/0x530
kthread+0x135/0x160
? set_kthread_struct+0x40/0x40
ret_from_fork+0x1f/0x30

which is due to holding the ctx->uring_lock when flushing existing
pending work, while the pending work flushing may need to grab the uring
lock if we're using IOPOLL.

Fix this by dropping the uring_lock a bit earlier as part of the flush.

Cc: stable@vger.kernel.org
Link: https://github.com/axboe/liburing/issues/404
Tested-by: Ammar Faizi <ammarfaizi2@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/io_uring.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -7166,17 +7166,19 @@ static int io_rsrc_ref_quiesce(struct io
/* kill initial ref, already quiesced if zero */
if (atomic_dec_and_test(&data->refs))
break;
+ mutex_unlock(&ctx->uring_lock);
flush_delayed_work(&ctx->rsrc_put_work);
ret = wait_for_completion_interruptible(&data->done);
- if (!ret)
+ if (!ret) {
+ mutex_lock(&ctx->uring_lock);
break;
+ }

atomic_inc(&data->refs);
/* wait for all works potentially completing data->done */
flush_delayed_work(&ctx->rsrc_put_work);
reinit_completion(&data->done);

- mutex_unlock(&ctx->uring_lock);
ret = io_run_task_work_sig();
mutex_lock(&ctx->uring_lock);