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From: Thomas Gleixner <tglx@linutronix.de>
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To: Peter Zijlstra <peterz@infradead.org>, Jens Axboe <axboe@kernel.dk>
Cc: Jann Horn <jannh@google.com>, Ingo Molnar <mingo@redhat.com>, Darren
 Hart <dvhart@infradead.org>, Davidlohr Bueso <dave@stgolabs.net>,
 =?utf-8?Q?Andr=C3=A9?=
 Almeida <andrealmeid@igalia.com>, kernel list
 <linux-kernel@vger.kernel.org>, Pavel Begunkov <asml.silence@gmail.com>,
 io-uring <io-uring@vger.kernel.org>
Subject: Re: futex+io_uring: futex_q::task can maybe be dangling (but is not
 actually accessed, so it's fine)
In-Reply-To: <20250113143832.GH5388@noisy.programming.kicks-ass.net>
References: <CAG48ez2HHU+vSCcurs5TsFXiEfUhLSXbEzcugBSTBZgBWkzpuA@mail.gmail.com>
 <3b78348b-a804-4072-b088-9519353edb10@kernel.dk>
 <20250113143832.GH5388@noisy.programming.kicks-ass.net>
Date: Wed, 15 Jan 2025 11:20:17 +0100
Message-ID: <877c6wcra6.ffs@tglx>
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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