From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>,
Jens Axboe <axboe@kernel.dk>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] block: switch to atomic_t for request references
Date: Wed, 8 Dec 2021 10:50:10 -0800 [thread overview]
Message-ID: <CAHk-=wg6reEPRY6ZDNA=3=cGRyK1csKhw0p3Ug57Z9by_Ev9Hw@mail.gmail.com> (raw)
In-Reply-To: <20211208184416.GY16608@worktop.programming.kicks-ass.net>
On Wed, Dec 8, 2021 at 10:44 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> From testing xadd had different flags from add; I've not yet looked at
> the SDM to see what it said on the matter.
That should not be the case. Just checked, and it just says
"The CF, PF, AF, SF, ZF, and OF flags are set according to the
result of the addition, which is stored in the destination operand"
which shows that I was confused about 'xadd' - I thought it returned
the old value in the register ("fetch_add"). It doesn't. It returns
the new one ("add_fetch"). And then 'fetch_add' ends up undoing it by
doing a sub or whatever.
So the actual returned value and the flags should match on x86.
Other architectures have the "return old value" model, which does mean
that my "different architectures can have different preferences for
which one to test" argument was right, even if I got xadd wrong.
Linus
next prev parent reply other threads:[~2021-12-08 18:50 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-03 15:35 [PATCH] block: switch to atomic_t for request references Jens Axboe
2021-12-03 15:56 ` Keith Busch
2021-12-06 6:53 ` Christoph Hellwig
2021-12-06 8:31 ` Peter Zijlstra
2021-12-06 16:32 ` Jens Axboe
2021-12-06 17:19 ` Peter Zijlstra
2021-12-06 17:35 ` Linus Torvalds
2021-12-06 18:13 ` Jens Axboe
2021-12-06 20:51 ` Kees Cook
2021-12-06 21:17 ` Linus Torvalds
2021-12-06 23:28 ` Kees Cook
2021-12-07 0:13 ` Linus Torvalds
2021-12-07 4:56 ` Kees Cook
2021-12-07 9:34 ` Peter Zijlstra
2021-12-07 16:03 ` Linus Torvalds
2021-12-07 10:30 ` Peter Zijlstra
2021-12-07 16:10 ` Linus Torvalds
2021-12-07 16:23 ` Peter Zijlstra
2021-12-06 16:31 ` Jens Axboe
2021-12-07 11:26 ` Peter Zijlstra
2021-12-07 13:28 ` Peter Zijlstra
2021-12-07 15:51 ` Peter Zijlstra
2021-12-07 16:13 ` Linus Torvalds
2021-12-07 16:52 ` Peter Zijlstra
2021-12-07 17:41 ` Peter Zijlstra
2021-12-07 17:43 ` Linus Torvalds
2021-12-07 17:45 ` Linus Torvalds
2021-12-07 20:28 ` Peter Zijlstra
2021-12-07 23:23 ` Linus Torvalds
2021-12-08 17:07 ` Peter Zijlstra
2021-12-08 18:00 ` Linus Torvalds
2021-12-08 18:44 ` Peter Zijlstra
2021-12-08 18:50 ` Linus Torvalds [this message]
2021-12-08 20:32 ` Peter Zijlstra
2021-12-10 10:57 ` Peter Zijlstra
2021-12-10 12:38 ` Peter Zijlstra
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