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

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Date:   Tue, 25 Apr 2023 12:58:12 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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 Tue, Apr 25, 2023 at 12:49 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> The last time this came up I shared your view however Mark argued for
> the READ_ONCE() thusly:
>
>   https://lore.kernel.org/all/Y71LoCIl+IFdy9D8@FVFF77S0Q05N/T/#u

Hmm.

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.

Ok. I'll try to remember this, but maybe it might be worth documenting.

Jens - I don't think this actually matters for the f_mode value issue,
since the only thing that might change is that FMODE_NOWAIT bit, but I
was clearly wrong on READ_ONCE(). So that loop should have it, just to
have the right pattern after all.

My bad.

             Linus