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* possible deadlock in __io_queue_deferred
@ 2020-08-10 15:36 syzbot
  2020-08-10 15:55 ` Jens Axboe
  0 siblings, 1 reply; 6+ messages in thread
From: syzbot @ 2020-08-10 15:36 UTC (permalink / raw)
  To: axboe, io-uring, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

Hello,

syzbot found the following issue on:

HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com

============================================
WARNING: possible recursive locking detected
5.8.0-syzkaller #0 Not tainted
--------------------------------------------
syz-executor287/6816 is trying to acquire lock:
ffff888093cdb4d8 (&ctx->completion_lock){....}-{2:2}, at: spin_lock_irq include/linux/spinlock.h:379 [inline]
ffff888093cdb4d8 (&ctx->completion_lock){....}-{2:2}, at: io_queue_linked_timeout fs/io_uring.c:5928 [inline]
ffff888093cdb4d8 (&ctx->completion_lock){....}-{2:2}, at: __io_queue_async_work fs/io_uring.c:1192 [inline]
ffff888093cdb4d8 (&ctx->completion_lock){....}-{2:2}, at: __io_queue_deferred+0x36a/0x790 fs/io_uring.c:1237

but task is already holding lock:
ffff888093cdb4d8 (&ctx->completion_lock){....}-{2:2}, at: io_cqring_overflow_flush+0xc6/0xab0 fs/io_uring.c:1333

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&ctx->completion_lock);
  lock(&ctx->completion_lock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

1 lock held by syz-executor287/6816:
 #0: ffff888093cdb4d8 (&ctx->completion_lock){....}-{2:2}, at: io_cqring_overflow_flush+0xc6/0xab0 fs/io_uring.c:1333

stack backtrace:
CPU: 1 PID: 6816 Comm: syz-executor287 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1f0/0x31e lib/dump_stack.c:118
 print_deadlock_bug kernel/locking/lockdep.c:2391 [inline]
 check_deadlock kernel/locking/lockdep.c:2432 [inline]
 validate_chain+0x69a4/0x88a0 kernel/locking/lockdep.c:3202
 __lock_acquire+0x1161/0x2ab0 kernel/locking/lockdep.c:4426
 lock_acquire+0x160/0x730 kernel/locking/lockdep.c:5005
 __raw_spin_lock_irq include/linux/spinlock_api_smp.h:128 [inline]
 _raw_spin_lock_irq+0x67/0x80 kernel/locking/spinlock.c:167
 spin_lock_irq include/linux/spinlock.h:379 [inline]
 io_queue_linked_timeout fs/io_uring.c:5928 [inline]
 __io_queue_async_work fs/io_uring.c:1192 [inline]
 __io_queue_deferred+0x36a/0x790 fs/io_uring.c:1237
 io_cqring_overflow_flush+0x774/0xab0 fs/io_uring.c:1359
 io_ring_ctx_wait_and_kill+0x2a1/0x570 fs/io_uring.c:7808
 io_uring_release+0x59/0x70 fs/io_uring.c:7829
 __fput+0x34f/0x7b0 fs/file_table.c:281
 task_work_run+0x137/0x1c0 kernel/task_work.c:135
 exit_task_work include/linux/task_work.h:25 [inline]
 do_exit+0x5f3/0x1f20 kernel/exit.c:806
 do_group_exit+0x161/0x2d0 kernel/exit.c:903
 __do_sys_exit_group+0x13/0x20 kernel/exit.c:914
 __se_sys_exit_group+0x10/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x37/0x40 kernel/exit.c:912
 do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x43f598
Code: Bad RIP value.
RSP: 002b:00007fffdac2bf58 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000000000043f598
RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
RBP: 00000000004beda8 R08: 00000000000000e7 R09: ffffffffffffffd0
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00000000006d11a0 R14: 0000000000000000 R15: 0000000000000000


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches

^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: possible deadlock in __io_queue_deferred
  2020-08-10 15:36 possible deadlock in __io_queue_deferred syzbot
@ 2020-08-10 15:55 ` Jens Axboe
  2020-08-11 14:00   ` Stefano Garzarella
  0 siblings, 1 reply; 6+ messages in thread
From: Jens Axboe @ 2020-08-10 15:55 UTC (permalink / raw)
  To: syzbot, io-uring, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

On 8/10/20 9:36 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com

Thanks, the below should fix this one.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 443eecdfeda9..f9be665d1c5e 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -898,6 +898,7 @@ static void io_put_req(struct io_kiocb *req);
 static void io_double_put_req(struct io_kiocb *req);
 static void __io_double_put_req(struct io_kiocb *req);
 static struct io_kiocb *io_prep_linked_timeout(struct io_kiocb *req);
+static void __io_queue_linked_timeout(struct io_kiocb *req);
 static void io_queue_linked_timeout(struct io_kiocb *req);
 static int __io_sqe_files_update(struct io_ring_ctx *ctx,
 				 struct io_uring_files_update *ip,
@@ -1179,7 +1180,7 @@ static void io_prep_async_link(struct io_kiocb *req)
 			io_prep_async_work(cur);
 }
 
-static void __io_queue_async_work(struct io_kiocb *req)
+static struct io_kiocb *__io_queue_async_work(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	struct io_kiocb *link = io_prep_linked_timeout(req);
@@ -1187,16 +1188,19 @@ static void __io_queue_async_work(struct io_kiocb *req)
 	trace_io_uring_queue_async_work(ctx, io_wq_is_hashed(&req->work), req,
 					&req->work, req->flags);
 	io_wq_enqueue(ctx->io_wq, &req->work);
-
-	if (link)
-		io_queue_linked_timeout(link);
+	return link;
 }
 
 static void io_queue_async_work(struct io_kiocb *req)
 {
+	struct io_kiocb *link;
+
 	/* init ->work of the whole link before punting */
 	io_prep_async_link(req);
-	__io_queue_async_work(req);
+	link = __io_queue_async_work(req);
+
+	if (link)
+		io_queue_linked_timeout(link);
 }
 
 static void io_kill_timeout(struct io_kiocb *req)
@@ -1229,12 +1233,19 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
 	do {
 		struct io_defer_entry *de = list_first_entry(&ctx->defer_list,
 						struct io_defer_entry, list);
+		struct io_kiocb *link;
 
 		if (req_need_defer(de->req, de->seq))
 			break;
 		list_del_init(&de->list);
 		/* punt-init is done before queueing for defer */
-		__io_queue_async_work(de->req);
+		link = __io_queue_async_work(de->req);
+		if (link) {
+			__io_queue_linked_timeout(link);
+			/* drop submission reference */
+			link->flags |= REQ_F_COMP_LOCKED;
+			io_put_req(link);
+		}
 		kfree(de);
 	} while (!list_empty(&ctx->defer_list));
 }
@@ -5945,15 +5956,12 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 	return HRTIMER_NORESTART;
 }
 
-static void io_queue_linked_timeout(struct io_kiocb *req)
+static void __io_queue_linked_timeout(struct io_kiocb *req)
 {
-	struct io_ring_ctx *ctx = req->ctx;
-
 	/*
 	 * If the list is now empty, then our linked request finished before
 	 * we got a chance to setup the timer
 	 */
-	spin_lock_irq(&ctx->completion_lock);
 	if (!list_empty(&req->link_list)) {
 		struct io_timeout_data *data = &req->io->timeout;
 
@@ -5961,6 +5969,14 @@ static void io_queue_linked_timeout(struct io_kiocb *req)
 		hrtimer_start(&data->timer, timespec64_to_ktime(data->ts),
 				data->mode);
 	}
+}
+
+static void io_queue_linked_timeout(struct io_kiocb *req)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+
+	spin_lock_irq(&ctx->completion_lock);
+	__io_queue_linked_timeout(req);
 	spin_unlock_irq(&ctx->completion_lock);
 
 	/* drop submission reference */

-- 
Jens Axboe


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: possible deadlock in __io_queue_deferred
  2020-08-10 15:55 ` Jens Axboe
@ 2020-08-11 14:00   ` Stefano Garzarella
  2020-08-11 14:21     ` Jens Axboe
  0 siblings, 1 reply; 6+ messages in thread
From: Stefano Garzarella @ 2020-08-11 14:00 UTC (permalink / raw)
  To: Jens Axboe
  Cc: syzbot, io-uring, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

On Mon, Aug 10, 2020 at 09:55:17AM -0600, Jens Axboe wrote:
> On 8/10/20 9:36 AM, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
> > dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
> > compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com
> 
> Thanks, the below should fix this one.

Yeah, it seems right to me, since only __io_queue_deferred() (invoked by
io_commit_cqring()) can be called with 'completion_lock' held.

Just out of curiosity, while exploring the code I noticed that we call
io_commit_cqring() always with the 'completion_lock' held, except in the
io_poll_* functions.

That's because then there can't be any concurrency?

Thanks,
Stefano

> 
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 443eecdfeda9..f9be665d1c5e 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -898,6 +898,7 @@ static void io_put_req(struct io_kiocb *req);
>  static void io_double_put_req(struct io_kiocb *req);
>  static void __io_double_put_req(struct io_kiocb *req);
>  static struct io_kiocb *io_prep_linked_timeout(struct io_kiocb *req);
> +static void __io_queue_linked_timeout(struct io_kiocb *req);
>  static void io_queue_linked_timeout(struct io_kiocb *req);
>  static int __io_sqe_files_update(struct io_ring_ctx *ctx,
>  				 struct io_uring_files_update *ip,
> @@ -1179,7 +1180,7 @@ static void io_prep_async_link(struct io_kiocb *req)
>  			io_prep_async_work(cur);
>  }
>  
> -static void __io_queue_async_work(struct io_kiocb *req)
> +static struct io_kiocb *__io_queue_async_work(struct io_kiocb *req)
>  {
>  	struct io_ring_ctx *ctx = req->ctx;
>  	struct io_kiocb *link = io_prep_linked_timeout(req);
> @@ -1187,16 +1188,19 @@ static void __io_queue_async_work(struct io_kiocb *req)
>  	trace_io_uring_queue_async_work(ctx, io_wq_is_hashed(&req->work), req,
>  					&req->work, req->flags);
>  	io_wq_enqueue(ctx->io_wq, &req->work);
> -
> -	if (link)
> -		io_queue_linked_timeout(link);
> +	return link;
>  }
>  
>  static void io_queue_async_work(struct io_kiocb *req)
>  {
> +	struct io_kiocb *link;
> +
>  	/* init ->work of the whole link before punting */
>  	io_prep_async_link(req);
> -	__io_queue_async_work(req);
> +	link = __io_queue_async_work(req);
> +
> +	if (link)
> +		io_queue_linked_timeout(link);
>  }
>  
>  static void io_kill_timeout(struct io_kiocb *req)
> @@ -1229,12 +1233,19 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
>  	do {
>  		struct io_defer_entry *de = list_first_entry(&ctx->defer_list,
>  						struct io_defer_entry, list);
> +		struct io_kiocb *link;
>  
>  		if (req_need_defer(de->req, de->seq))
>  			break;
>  		list_del_init(&de->list);
>  		/* punt-init is done before queueing for defer */
> -		__io_queue_async_work(de->req);
> +		link = __io_queue_async_work(de->req);
> +		if (link) {
> +			__io_queue_linked_timeout(link);
> +			/* drop submission reference */
> +			link->flags |= REQ_F_COMP_LOCKED;
> +			io_put_req(link);
> +		}
>  		kfree(de);
>  	} while (!list_empty(&ctx->defer_list));
>  }
> @@ -5945,15 +5956,12 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
>  	return HRTIMER_NORESTART;
>  }
>  
> -static void io_queue_linked_timeout(struct io_kiocb *req)
> +static void __io_queue_linked_timeout(struct io_kiocb *req)
>  {
> -	struct io_ring_ctx *ctx = req->ctx;
> -
>  	/*
>  	 * If the list is now empty, then our linked request finished before
>  	 * we got a chance to setup the timer
>  	 */
> -	spin_lock_irq(&ctx->completion_lock);
>  	if (!list_empty(&req->link_list)) {
>  		struct io_timeout_data *data = &req->io->timeout;
>  
> @@ -5961,6 +5969,14 @@ static void io_queue_linked_timeout(struct io_kiocb *req)
>  		hrtimer_start(&data->timer, timespec64_to_ktime(data->ts),
>  				data->mode);
>  	}
> +}
> +
> +static void io_queue_linked_timeout(struct io_kiocb *req)
> +{
> +	struct io_ring_ctx *ctx = req->ctx;
> +
> +	spin_lock_irq(&ctx->completion_lock);
> +	__io_queue_linked_timeout(req);
>  	spin_unlock_irq(&ctx->completion_lock);
>  
>  	/* drop submission reference */
> 
> -- 
> Jens Axboe
> 


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: possible deadlock in __io_queue_deferred
  2020-08-11 14:00   ` Stefano Garzarella
@ 2020-08-11 14:21     ` Jens Axboe
  2020-08-11 14:44       ` Stefano Garzarella
  0 siblings, 1 reply; 6+ messages in thread
From: Jens Axboe @ 2020-08-11 14:21 UTC (permalink / raw)
  To: Stefano Garzarella
  Cc: syzbot, io-uring, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

On 8/11/20 8:00 AM, Stefano Garzarella wrote:
> On Mon, Aug 10, 2020 at 09:55:17AM -0600, Jens Axboe wrote:
>> On 8/10/20 9:36 AM, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
>>> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com
>>
>> Thanks, the below should fix this one.
> 
> Yeah, it seems right to me, since only __io_queue_deferred() (invoked by
> io_commit_cqring()) can be called with 'completion_lock' held.

Right

> Just out of curiosity, while exploring the code I noticed that we call
> io_commit_cqring() always with the 'completion_lock' held, except in the
> io_poll_* functions.
> 
> That's because then there can't be any concurrency?

Do you mean the iopoll functions? Because we're definitely holding it
for the io_poll_* functions.

-- 
Jens Axboe


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: possible deadlock in __io_queue_deferred
  2020-08-11 14:21     ` Jens Axboe
@ 2020-08-11 14:44       ` Stefano Garzarella
  2020-08-11 14:45         ` Jens Axboe
  0 siblings, 1 reply; 6+ messages in thread
From: Stefano Garzarella @ 2020-08-11 14:44 UTC (permalink / raw)
  To: Jens Axboe
  Cc: syzbot, io-uring, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

On Tue, Aug 11, 2020 at 08:21:12AM -0600, Jens Axboe wrote:
> On 8/11/20 8:00 AM, Stefano Garzarella wrote:
> > On Mon, Aug 10, 2020 at 09:55:17AM -0600, Jens Axboe wrote:
> >> On 8/10/20 9:36 AM, syzbot wrote:
> >>> Hello,
> >>>
> >>> syzbot found the following issue on:
> >>>
> >>> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
> >>> git tree:       upstream
> >>> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
> >>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
> >>> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
> >>> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> >>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
> >>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
> >>>
> >>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >>> Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com
> >>
> >> Thanks, the below should fix this one.
> > 
> > Yeah, it seems right to me, since only __io_queue_deferred() (invoked by
> > io_commit_cqring()) can be called with 'completion_lock' held.
> 
> Right
> 
> > Just out of curiosity, while exploring the code I noticed that we call
> > io_commit_cqring() always with the 'completion_lock' held, except in the
> > io_poll_* functions.
> > 
> > That's because then there can't be any concurrency?
> 
> Do you mean the iopoll functions? Because we're definitely holding it
> for the io_poll_* functions.

Right, the only one seems io_iopoll_complete().

So, IIUC, in this case we are actively polling the level below,
so there shouldn't be any asynchronous events, is it right?

Thanks,
Stefano


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: possible deadlock in __io_queue_deferred
  2020-08-11 14:44       ` Stefano Garzarella
@ 2020-08-11 14:45         ` Jens Axboe
  0 siblings, 0 replies; 6+ messages in thread
From: Jens Axboe @ 2020-08-11 14:45 UTC (permalink / raw)
  To: Stefano Garzarella
  Cc: syzbot, io-uring, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

On 8/11/20 8:44 AM, Stefano Garzarella wrote:
> On Tue, Aug 11, 2020 at 08:21:12AM -0600, Jens Axboe wrote:
>> On 8/11/20 8:00 AM, Stefano Garzarella wrote:
>>> On Mon, Aug 10, 2020 at 09:55:17AM -0600, Jens Axboe wrote:
>>>> On 8/10/20 9:36 AM, syzbot wrote:
>>>>> Hello,
>>>>>
>>>>> syzbot found the following issue on:
>>>>>
>>>>> HEAD commit:    449dc8c9 Merge tag 'for-v5.9' of git://git.kernel.org/pub/..
>>>>> git tree:       upstream
>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=14d41e02900000
>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9d25235bf0162fbc
>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=996f91b6ec3812c48042
>>>>> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
>>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=133c9006900000
>>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1191cb1a900000
>>>>>
>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>>> Reported-by: syzbot+996f91b6ec3812c48042@syzkaller.appspotmail.com
>>>>
>>>> Thanks, the below should fix this one.
>>>
>>> Yeah, it seems right to me, since only __io_queue_deferred() (invoked by
>>> io_commit_cqring()) can be called with 'completion_lock' held.
>>
>> Right
>>
>>> Just out of curiosity, while exploring the code I noticed that we call
>>> io_commit_cqring() always with the 'completion_lock' held, except in the
>>> io_poll_* functions.
>>>
>>> That's because then there can't be any concurrency?
>>
>> Do you mean the iopoll functions? Because we're definitely holding it
>> for the io_poll_* functions.
> 
> Right, the only one seems io_iopoll_complete().
> 
> So, IIUC, in this case we are actively polling the level below,
> so there shouldn't be any asynchronous events, is it right?

Right, that's serialized by itself.

-- 
Jens Axboe


^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2020-08-11 14:45 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-08-10 15:36 possible deadlock in __io_queue_deferred syzbot
2020-08-10 15:55 ` Jens Axboe
2020-08-11 14:00   ` Stefano Garzarella
2020-08-11 14:21     ` Jens Axboe
2020-08-11 14:44       ` Stefano Garzarella
2020-08-11 14:45         ` Jens Axboe

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