From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Daniel Xu <dxu@dxuuu.xyz>,
mingo@redhat.com, will@kernel.org, longman@redhat.com,
boqun.feng@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] seqlock: Use WRITE_ONCE() when updating sequence
Date: Wed, 18 Dec 2024 17:23:25 +0100 [thread overview]
Message-ID: <20241218162325.GH2354@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <ff2aecbc-cd5d-4a9c-92b1-792555300301@paulmck-laptop>
On Wed, Dec 18, 2024 at 07:43:41AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 18, 2024 at 11:30:00AM +0100, Peter Zijlstra wrote:
> > On Tue, Dec 17, 2024 at 03:17:36PM -0800, Daniel Xu wrote:
> > > @@ -405,7 +405,7 @@ do { \
> > > static inline void do_raw_write_seqcount_begin(seqcount_t *s)
> > > {
> > > kcsan_nestable_atomic_begin();
> > > - s->sequence++;
> > > + WRITE_ONCE(s->sequence, READ_ONCE(s->sequence) + 1);
> > > smp_wmb();
> > > }
> >
> > This results in significantly worse code-gen, it will change an inc to
> > memory with a load,inc,store.
>
> Isn't that code-generation bug in the process of being fixed?
Last time I looked the compiler wasn't allowed to touch it because of
all the volatile going around. Did anything change?
> And, either way, given the likely cache miss, should we really care?
Yeah, extra register pressure too.
next prev parent reply other threads:[~2024-12-18 16:23 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-12-17 23:17 [PATCH] seqlock: Use WRITE_ONCE() when updating sequence Daniel Xu
2024-12-18 3:30 ` Waiman Long
2024-12-18 15:45 ` Paul E. McKenney
2024-12-18 16:10 ` Waiman Long
2024-12-18 16:57 ` Paul E. McKenney
2024-12-18 18:38 ` Waiman Long
2024-12-18 18:52 ` Paul E. McKenney
2024-12-18 20:09 ` Waiman Long
2024-12-18 10:30 ` Peter Zijlstra
2024-12-18 15:43 ` Paul E. McKenney
2024-12-18 16:23 ` Peter Zijlstra [this message]
2024-12-18 16:29 ` Peter Zijlstra
2024-12-18 16:59 ` Paul E. McKenney
2024-12-18 17:12 ` Peter Zijlstra
2024-12-18 19:15 ` Paul E. McKenney
2024-12-18 19:56 ` Florian Weimer
2024-12-19 16:10 ` Paul E. McKenney
2024-12-19 16:45 ` Florian Weimer
2024-12-19 17:48 ` Paul E. McKenney
2024-12-19 17:53 ` Peter Zijlstra
2024-12-19 16:34 ` Will Deacon
2024-12-19 17:53 ` Paul E. McKenney
2024-12-19 17:58 ` Will Deacon
2024-12-19 18:06 ` Paul E. McKenney
2024-12-19 18:31 ` Will Deacon
2024-12-20 17:40 ` Paul E. McKenney
2025-01-25 0:31 ` Daniel Xu
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