linux-kernel - Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

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Message-ID: <20250305045617.3038-1-hdanton@sina.com>
Date: Wed,  5 Mar 2025 12:56:16 +0800
From: Hillf Danton <hdanton@...a.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>,
	Mateusz Guzik <mjguzik@...il.com>,
	"Sapkal, Swapnil" <swapnil.sapkal@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

On Wed, 5 Mar 2025 00:49:09 +0100 Oleg Nesterov <oleg@...hat.com>
> 
> Of course! Again, whatever the woken writer checks in pipe_writable()
> lockless, another writer can make pipe_full() true again.
> 
> But why do we care? Why do you think that the change you propose makes

Because of the hang reported.

> more sense than the fix from Prateek or the (already merged) Linus's fix?
> 
See the loop in  ___wait_event(),

	for (;;) {
		prepare_to_wait_event();

		// flip
		if (condition)
			break;

		schedule();
	}

After wakeup, waiter will sleep again if condition flips false on the waker
side before waiter checks condition, even if condition is atomic, no?