linux-kernel - Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

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Message-ID: <20241228163231.GA19293@redhat.com>
Date: Sat, 28 Dec 2024 17:32:32 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Manfred Spraul <manfred@...orfullife.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, viro@...iv.linux.org.uk,
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Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping
 processes during pipe read/write

On 12/28, Oleg Nesterov wrote:
>
> If nothing else, consider
>
> 	int CONDITION;
> 	wait_queue_head_t WQ;
>
> 	void wake(void)
> 	{
> 		CONDITION = 1;
> 		wake_up(WQ);
> 	}
>
> 	void wait(void)
> 	{
> 		DEFINE_WAIT_FUNC(entry, woken_wake_function);
>
> 		add_wait_queue(WQ, entry);
> 		if (!CONDITION)
> 			wait_woken(entry, ...);
> 		remove_wait_queue(WQ, entry);
> 	}
>
> this code is correct even if LOAD(CONDITION) can leak into the critical
> section in add_wait_queue(), so CPU running wait() can actually do
>
> 		// add_wait_queue
> 		spin_lock(WQ->lock);
> 		LOAD(CONDITION);	// false!
> 		list_add(entry, head);
> 		spin_unlock(WQ->lock);
>
> 		if (!false)		// result of the LOAD above
> 			wait_woken(entry, ...);
>
> Now suppose that another CPU executes wake() between LOAD(CONDITION)
> and list_add(entry, head). With your patch wait() will miss the event.
> The same for __pollwait(), I think...
>
> No?

Even simpler,

	void wait(void)
	{
		DEFINE_WAIT(entry);

		__set_current_state(XXX);
		add_wait_queue(WQ, entry);

		if (!CONDITION)
			schedule();

		remove_wait_queue(WQ, entry);
		__set_current_state(TASK_RUNNING);
	}

This code is ugly but currently correct unless I am totally confused.

Oleg.