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Date:   Thu, 3 Aug 2023 20:02:03 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Mateusz Guzik <mjguzik@gmail.com>, Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@lst.de>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Seth Forshee <sforshee@kernel.org>,
        linux-fsdevel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] file: always lock position
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On Thu, Aug 03, 2023 at 08:45:54AM -0700, Linus Torvalds wrote:
> On Thu, 3 Aug 2023 at 02:53, Mateusz Guzik <mjguzik@gmail.com> wrote:
> >
> > So yes, atomics remain expensive on x86-64 even on a very moden uarch
> > and their impact is measurable in a syscall like read.
> 
> Well, a patch like this should fix it.
> 
> I intentionally didn't bother with the alpha osf version of readdir,
> because nobody cares, but I guess we could do this in the header too.
> 
> Or we could have split the FMODE_ATOMIC_POS bit into two, and had a
> "ALWAYS" version and a regular version, but just having a
> "fdget_dir()" made it simpler.
> 
> So this - together with just reverting commit 20ea1e7d13c1 ("file:
> always lock position for FMODE_ATOMIC_POS") - *should* fix any
> performance regression.
> 
> But I have not tested it at all. So....

Yeah, this is my suggestion - and your earlier suggestion - in the
thread. Only thing that's missing is exclusion with seek on directories
as that's the heinous part.