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

[Greg Kroah-Hartman]

From: Jens Axboe <axboe@xxxxxxxxx>

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@xxxxxxxxxxxxxxx
Link: https://github.com/axboe/liburing/issues/404
Tested-by: Ammar Faizi <ammarfaizi2@xxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 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);
 	} while (ret >= 0);