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

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Message-ID: <20250305224648.3058-1-hdanton@sina.com>
Date: Thu,  6 Mar 2025 06:46:45 +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 12:44:34 +0100 Oleg Nesterov <oleg@...hat.com>
> On 03/05, Hillf Danton wrote:
> > 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?
> 
> Yes, but in this case pipe_full() == true is correct, this writer can
> safely sleep.
> 
No, because no reader is woken up before sleep to make pipe not full.