From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Oleg Nesterov <oleg@redhat.com>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Michael Ellerman <mpe@ellerman.id.au>,
linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
Ingo Molnar <mingo@kernel.org>,
Alan Stern <stern@rowland.harvard.edu>
Subject: Re: Question on smp_mb__before_spinlock
Date: Mon, 5 Sep 2016 11:10:22 +0100 [thread overview]
Message-ID: <20160905101021.GC2649@arm.com> (raw)
In-Reply-To: <20160905093753.GN10138@twins.programming.kicks-ass.net>
On Mon, Sep 05, 2016 at 11:37:53AM +0200, Peter Zijlstra wrote:
> So recently I've had two separate issues that touched upon
> smp_mb__before_spinlock().
>
>
> Since its inception, our understanding of ACQUIRE, esp. as applied to
> spinlocks, has changed somewhat. Also, I wonder if, with a simple
> change, we cannot make it provide more.
>
> The problem with the comment is that the STORE done by spin_lock isn't
> itself ordered by the ACQUIRE, and therefore a later LOAD can pass over
> it and cross with any prior STORE, rendering the default WMB
> insufficient (pointed out by Alan).
>
> Now, this is only really a problem on PowerPC and ARM64, the former of
> which already defined smp_mb__before_spinlock() as a smp_mb(), the
> latter does not, Will?
I just replied to that thread and, assuming I've groked the sched/core.c
usage correctly, then it does look like we need to make that an smp_mb()
with the current code.
> The second issue I wondered about is spinlock transitivity. All except
> powerpc have RCsc locks, and since Power already does a full mb, would
> it not make sense to put it _after_ the spin_lock(), which would provide
> the same guarantee, but also upgrades the section to RCsc.
>
> That would make all schedule() calls fully transitive against one
> another.
It would also match the way in which the arm64 atomic_*_return ops
are implemented, since full barrier semantics are required there.
> That is, would something like the below make sense?
Works for me, but I'll do a fix to smp_mb__before_spinlock anyway for
the stable tree.
The only slight annoyance is that, on arm64 anyway, a store-release
appearing in program order before the LOCK operation will be observed
in order, so if the write of CONDITION=1 in the try_to_wake_up case
used smp_store_release, we wouldn't need this barrier at all.
Will
next prev parent reply other threads:[~2016-09-05 10:10 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-05 9:37 Question on smp_mb__before_spinlock Peter Zijlstra
2016-09-05 9:56 ` kbuild test robot
2016-09-05 10:19 ` Peter Zijlstra
2016-09-05 11:26 ` Fengguang Wu
2016-09-05 10:10 ` Will Deacon [this message]
2016-09-06 11:17 ` Peter Zijlstra
2016-09-06 17:42 ` Will Deacon
2016-09-05 10:37 ` Paul E. McKenney
2016-09-05 11:34 ` Peter Zijlstra
2016-09-05 13:57 ` Paul E. McKenney
2016-09-05 10:51 ` kbuild test robot
2016-09-07 12:17 ` Nicholas Piggin
2016-09-07 13:23 ` Peter Zijlstra
2016-09-07 13:51 ` Will Deacon
2016-09-12 2:35 ` Nicholas Piggin
2016-09-12 2:27 ` Nicholas Piggin
2016-09-12 12:54 ` Peter Zijlstra
2016-09-13 2:05 ` Nicholas Piggin
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