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