From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Peter Anvin <hpa@zytor.com>,
Andrea Parri <parri.andrea@gmail.com>,
Ingo Molnar <mingo@kernel.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
Vince Weaver <vincent.weaver@maine.edu>,
Thomas Gleixner <tglx@linutronix.de>,
Jiri Olsa <jolsa@redhat.com>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Stephane Eranian <eranian@google.com>,
Will Deacon <will@kernel.org>,
linux-tip-commits@vger.kernel.org
Subject: Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Thu, 9 Sep 2021 09:25:30 +0200 [thread overview]
Message-ID: <YTm26u9i3hpjrNpr@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHk-=wiXJygbW+_1BdSX6M8j6z4w8gRSHVcaD5saihaNJApnoQ@mail.gmail.com>
On Wed, Sep 08, 2021 at 09:08:33AM -0700, Linus Torvalds wrote:
> So if this is purely a RISC-V thing,
Just to clarify, I think the current RISC-V thing is stonger than
PowerPC, but maybe not as strong as say ARM64, but RISC-V memory
ordering is still somewhat hazy to me.
Specifically, the sequence:
/* critical section s */
WRITE_ONCE(x, 1);
FENCE RW, W
WRITE_ONCE(s.lock, 0); /* store S */
AMOSWAP %0, 1, r.lock /* store R */
FENCE R, RW
WRITE_ONCE(y, 1);
/* critical section r */
fully separates section s from section r, as in RW->RW ordering
(possibly not as strong as smp_mb() though), while on PowerPC it would
only impose TSO ordering between sections.
The AMOSWAP is a RmW and as such matches the W from the RW->W fence,
similarly it marches the R from the R->RW fence, yielding an:
RW-> W
RmW
R ->RW
ordering. It's the stores S and R that can be re-ordered, but not the
sections themselves (same on PowerPC and many others).
Clarification from a RISC-V enabled person would be appreciated.
> then I think it's entirely reasonable to
>
> spin_unlock(&r);
> spin_lock(&s);
>
> cannot be reordered.
I'm obviously completely in favour of that :-)
next prev parent reply other threads:[~2021-09-09 7:28 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-26 18:28 [PATCH memory-model 0/5] Updates to the formal memory model Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 1/5] tools/memory-model: Add litmus-test naming scheme Paul E. McKenney
2018-10-02 10:10 ` [tip:locking/core] " tip-bot for Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 2/5] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Paul E. McKenney
2018-10-02 10:11 ` [tip:locking/core] " tip-bot for Alan Stern
2021-09-08 11:00 ` Peter Zijlstra
2021-09-08 11:44 ` Peter Zijlstra
2021-09-08 14:42 ` Alan Stern
2021-09-08 15:12 ` Peter Zijlstra
2021-09-08 16:08 ` Linus Torvalds
2021-09-09 7:25 ` Peter Zijlstra [this message]
2021-09-09 13:35 ` Will Deacon
2021-09-09 17:02 ` Linus Torvalds
2021-09-09 18:59 ` Alan Stern
2021-09-09 17:03 ` Dan Lustig
2021-09-09 18:00 ` Paul E. McKenney
2021-09-10 14:20 ` Boqun Feng
2021-09-10 15:33 ` Palmer Dabbelt
2021-09-10 16:36 ` Alan Stern
2021-09-10 17:12 ` Peter Zijlstra
2021-09-10 17:56 ` Alan Stern
2021-09-10 17:17 ` Peter Zijlstra
2021-09-12 0:26 ` Boqun Feng
2021-09-10 0:01 ` Boqun Feng
2021-09-10 5:37 ` Boqun Feng
2021-09-10 9:33 ` Peter Zijlstra
2021-09-10 10:04 ` Boqun Feng
2021-09-10 13:48 ` Dan Lustig
2021-09-10 14:15 ` Boqun Feng
2021-09-09 17:46 ` Paul E. McKenney
2021-09-10 11:08 ` Will Deacon
2021-09-17 3:21 ` Nicholas Piggin
2021-09-17 5:31 ` Nicholas Piggin
2021-09-17 14:36 ` Michael Ellerman
2018-09-26 18:29 ` [PATCH memory-model 3/5] tools/memory-model: Fix a README typo Paul E. McKenney
2018-10-02 10:11 ` [tip:locking/core] " tip-bot for SeongJae Park
2018-09-26 18:29 ` [PATCH memory-model 4/5] tools/memory-model: Add more LKMM limitations Paul E. McKenney
2018-10-02 10:12 ` [tip:locking/core] " tip-bot for Paul E. McKenney
2018-09-26 18:29 ` [PATCH memory-model 5/5] doc: Replace smp_cond_acquire() with smp_cond_load_acquire() Paul E. McKenney
2018-10-02 10:12 ` [tip:locking/core] locking/memory-barriers: " tip-bot for Andrea Parri
2018-10-02 8:28 ` [PATCH memory-model 0/5] Updates to the formal memory model Ingo Molnar
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