linux-kernel - Re: futex+io_uring: futex_q::task can maybe be dangling (but is not actually accessed, so it's fine)

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Message-ID: <877c6wcra6.ffs@tglx>
Date: Wed, 15 Jan 2025 11:20:17 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>, Jens Axboe <axboe@...nel.dk>
Cc: Jann Horn <jannh@...gle.com>, Ingo Molnar <mingo@...hat.com>, Darren
 Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>,
 André
 Almeida <andrealmeid@...lia.com>, kernel list
 <linux-kernel@...r.kernel.org>, Pavel Begunkov <asml.silence@...il.com>,
 io-uring <io-uring@...r.kernel.org>
Subject: Re: futex+io_uring: futex_q::task can maybe be dangling (but is not
 actually accessed, so it's fine)

On Mon, Jan 13 2025 at 15:38, Peter Zijlstra wrote:
> On Fri, Jan 10, 2025 at 08:33:34PM -0700, Jens Axboe wrote:
>
>> @@ -548,7 +549,7 @@ void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
>>  
>>  	plist_node_init(&q->list, prio);
>>  	plist_add(&q->list, &hb->chain);
>> -	q->task = current;
>> +	q->task = task;
>>  }
>>  
>>  /**
>
> The alternative is, I suppose, to move the q->task assignment out to
> these two callsites instead. Thomas, any opinions?

That's fine as long as hb->lock is held, but the explicit argument makes
all of this simpler to understand.

Though I'm not really a fan of this part:

> +		__futex_queue(&ifd->q, hb, NULL);
> +		spin_unlock(&hb->lock);

Can we please add that @task argument to futex_queue() and keep the
internals in the futex code instead of pulling more stuff into io_uring?

Thanks,

        tglx