From: Peter Zijlstra <peterz@infradead.org>
To: Muchun Song <songmuchun@bytedance.com>
Cc: Will Deacon <will@kernel.org>,
mingo@redhat.com, mingo@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [External] Re: [PATCH] seqlock: Use while instead of if+goto in __read_seqcount_begin
Date: Wed, 15 Apr 2020 13:41:54 +0200 [thread overview]
Message-ID: <20200415114154.GF20730@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAMZfGtWk+gG7Z2bOy_bq=XnuuSJzic16zpeajhJiiekpJEFrFg@mail.gmail.com>
On Tue, Apr 14, 2020 at 08:01:06PM +0800, Muchun Song wrote:
> On Tue, Apr 14, 2020 at 7:05 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > .... That said,
> > Will, would it make sense to use smp_cond_load_relaxed() here ?
>
> I have a similar idea. Would it make sense to use smp_cond_load_acquire()
> in raw_read_seqcount_begin()?
Not sure; I did consider it, but that rmb it has seems more natural in
the over-all ordering scheme here. I mean:
load seqcount inc seqcount
rmb wmb
// load stuff // modify stuff
rmb wmb
compare seqcount inc seqcount
is nice and symmetric, making that upper left rmb an acquire 'works' but
is just weird IMO. And I suppose you can make the lower right wmb a
store-release, which is somewhat better, but then it gets all weird when
you consider things like barrier and latch.
So best to just leave it as is I think.
Those incs do seem to be really wanting a WRITE_ONCE() though.
next prev parent reply other threads:[~2020-04-15 11:44 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-09 13:45 [PATCH] seqlock: Use while instead of if+goto in __read_seqcount_begin Muchun Song
2020-04-10 11:56 ` Will Deacon
2020-04-14 8:58 ` [External] " Muchun Song
2020-04-14 11:05 ` Peter Zijlstra
2020-04-14 12:01 ` [External] " Muchun Song
2020-04-15 11:41 ` Peter Zijlstra [this message]
2020-04-14 13:48 ` Will Deacon
2020-04-15 11:44 ` Peter Zijlstra
2020-04-15 9:37 ` David Laight
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