linux-kernel - Re: [GIT PULL] pipe: nonblocking rw for io

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Message-ID: <20230508083929.GT83892@hirez.programming.kicks-ass.net>
Date:   Mon, 8 May 2023 10:39:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [GIT PULL] pipe: nonblocking rw for io_uring

On Sun, May 07, 2023 at 04:04:23PM +0200, Jonas Oberhauser wrote:
> 
> Am 4/25/2023 um 9:58 PM schrieb Linus Torvalds:
> > Yes, I think Mark is right. It's not that 'old' might be wrong - that
> > doesn't matter because cmpxchg will work it out - it's just that 'new'
> > might not be consistent with the old value we then use.
> 
> In the general pattern, besides the potential issue raised by Mark, tearing
> may also be an issue (longer example inspired by a case we met at the end of
> the mail) where 'old' being wrong matters.

There is yet another pattern where it actually matters:

	old = READ_ONCE(*ptr);
	do {
		if (cond(old))
			return false;

		new = func(old);
	} while (!try_cmpxchg(ptr, &old, new));

	return true;

In this case we rely on old being 'coherent'. The more obvious case is
where it returns old (also not uncommon), but even if it just checks a
(multi-bit) condition on old you don't want tearing.