From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Marco Elver <elver@google.com>, Will Deacon <will@kernel.org>,
paulmck <paulmck@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Segher Boessenkool <segher@kernel.crashing.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Alan Stern <stern@rowland.harvard.edu>,
Andrea Parri <parri.andrea@gmail.com>,
Boqun Feng <boqun.feng@gmail.com>,
Nicholas Piggin <npiggin@gmail.com>,
David Howells <dhowells@redhat.com>,
j alglave <j.alglave@ucl.ac.uk>,
luc maranget <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
linux-toolchains <linux-toolchains@vger.kernel.org>,
linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency
Date: Fri, 1 Oct 2021 11:18:37 -0700 [thread overview]
Message-ID: <CAHk-=whi_B36yw9Haw3sfSQhF7+Y1=bn_y2S=DwZ533yuF=izw@mail.gmail.com> (raw)
In-Reply-To: <1097444747.48074.1633109281556.JavaMail.zimbra@efficios.com>
On Fri, Oct 1, 2021 at 10:28 AM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> I've spent some quality time staring at generated assembler diff in the past
> days, and looking for code patterns of refcount_dec_and_test users, without
> much success. There are some cases which end up working by chance, e.g. in
> cases where the if leg has a smp_acquire__after_ctrl_dep and the else leg has
> code that emits a barrier(), but I did not find any buggy generated
> code per se. In order to observe those issues in real life, we would
> really need to have identical then/else legs to the branch.
Yeah, that's been very much my feeling too during this whole
discussion (including, very much, earlier threads).
All the examples about this being a problem are those kinds of
"identical or near-identical if/else statements" and they just don't
seem to be all that realistic.
Because immediately when the if-statement actually does something
_meaningful_, it just turns into a branch. And when people use atomics
- even the weak READ/WRITE_ONCE() kinds of things, never mind anything
stronger - it really doesn't give the compiler the option to move
things around all that much.
There's a reason the source code uses an if-statement, after all: that
is literally the logical code flow, and people write a very particular
dependency chain that is just very fundamental.
Having essentially the same thing on both sides just isn't a realistic
thing to do, and if it were - and you cared about performance in that
case, which is what this is all about, after all - you'd write it very
differently.
Linus
next prev parent reply other threads:[~2021-10-01 18:19 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-28 21:15 [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency Mathieu Desnoyers
2021-09-29 12:06 ` Marco Elver
2021-10-01 15:45 ` Mathieu Desnoyers
2021-10-01 16:20 ` Linus Torvalds
2021-10-01 17:28 ` Mathieu Desnoyers
2021-10-01 18:18 ` Linus Torvalds [this message]
2021-09-29 12:28 ` Florian Weimer
2021-09-29 17:41 ` Segher Boessenkool
2021-09-29 19:46 ` Florian Weimer
2021-10-01 16:13 ` Mathieu Desnoyers
2021-10-01 16:26 ` Florian Weimer
2021-10-01 16:35 ` Linus Torvalds
2021-10-10 14:02 ` Florian Weimer
2021-10-14 0:01 ` Paul E. McKenney
2021-10-14 2:14 ` Alan Stern
2021-10-14 16:14 ` Paul E. McKenney
2021-10-14 15:58 ` Florian Weimer
2021-10-14 16:23 ` Paul E. McKenney
2021-10-14 18:19 ` Florian Weimer
2021-10-14 21:09 ` Paul E. McKenney
2021-10-14 22:36 ` Linus Torvalds
2021-09-30 13:28 ` Mathieu Desnoyers
2021-09-29 14:47 ` Linus Torvalds
2021-09-29 14:54 ` Linus Torvalds
2021-09-29 19:50 ` Mathieu Desnoyers
2021-09-29 20:13 ` Mathieu Desnoyers
2021-09-29 19:27 ` Mathieu Desnoyers
2021-09-29 22:14 ` Linus Torvalds
2021-09-29 21:47 ` Segher Boessenkool
2021-09-29 23:57 ` Paul E. McKenney
2021-10-01 15:28 ` Mathieu Desnoyers
2021-10-01 22:53 ` Paul E. McKenney
2021-10-01 19:10 ` Segher Boessenkool
2021-10-01 22:50 ` Paul E. McKenney
2021-10-02 14:29 ` Alan Stern
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