linux-kernel - spin_lock implicit/explicit memory barrier

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Message-ID: <abcae6c7-6269-94e0-3e66-b7d07fbe10dc@colorfullife.com>
Date:	Tue, 9 Aug 2016 20:52:27 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Susi Sonnenschein <1vier1@....de>
Subject: spin_lock implicit/explicit memory barrier

Hi Benjamin, Hi Michael,

regarding commit 51d7d5205d33 ("powerpc: Add smp_mb() to 
arch_spin_is_locked()"):

For the ipc/sem code, I would like to replace the spin_is_locked() with 
a smp_load_acquire(), see:

http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n367

http://www.ozlabs.org/~akpm/mmots/broken-out/ipc-semc-fix-complex_count-vs-simple-op-race.patch

To my understanding, I must now add a smp_mb(), otherwise it would be 
broken on PowerPC:

The approach that the memory barrier is added into spin_is_locked() 
doesn't work because the code doesn't use spin_is_locked().

Correct?

--

     Manfred