linux-kernel - Re: [PATCH] task_work: Consume only item at a time while invoking the callbacks.

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Message-ID: <20250226125048.GC8995@redhat.com>
Date: Wed, 26 Feb 2025 13:50:48 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <longman@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] task_work: Consume only item at a time while invoking
 the callbacks.

On 02/21, Sebastian Andrzej Siewior wrote:
>
> Yi and syzbot managed to hang the task within task_run().
>
> The problem is
>  task_work_run() -> __fput() -> perf_release() ->
>  perf_event_release_kernel() -> _free_event() ->
>  perf_pending_task_sync() -> task_work_cancel() failed ->
>  rcuwait_wait_event().
>
> Once task_work_run() is running, the list of callbacks removed from the
> task_struct and from this point on task_work_cancel() can't remove any
> pending and not yet started work items.

But can this patch really solve the problem?

Suppose we have two tasks, T1 and T2.

T1 does fd = perf_event_open(pid => T2->pid);
T2 does fd = perf_event_open(pid => T1->pid);

Now, both T1 and T2 do close(fd), call task_work_run(), dequeue the
____fput work, and finally call __fput(). Now suppose that both perf
events fire before T1/T2 call perf_event_release_kernel/_free_event.

Now, T1 and T2 will hang forever in perf_pending_task_sync() waiting
for each other. task_work_cancel(current) can't succeed with or without
this patch.

No?

Oleg.