linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Message-ID: <1437023004.28088.27.camel@kernel.crashing.org>
Date:	Thu, 16 Jul 2015 15:03:24 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Will Deacon <will.deacon@....com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Thu, 2015-07-16 at 12:00 +1000, Michael Ellerman wrote:
> That would fix the problem with smp_mb__after_unlock_lock(), but not
> the original worry we had about loads happening before the SC in lock.

However I think isync fixes *that* :-) The problem with isync is as you
said, it's not a -memory- barrier per-se, it's an execution barrier /
context synchronizing instruction. The combination stwcx. + bne + isync
however prevents the execution of anything past the isync until the
stwcx has completed and the bne has been "decided", which prevents loads
from leaking into the LL/SC loop. It will also prevent a store in the
lock from being issued before the stwcx. has completed. It does *not*
prevent as far as I can tell another unrelated store before the lock
from leaking into the lock, including the one used to unlock a different
lock.

Cheers,
Ben.

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