linux-kernel - Clarification needed regarding memory barrier

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Message-ID: <CAPyr0yLsDXpoA41=Y6PP-iVRnP_cGh-jmA1sDYECU1ZdCTwCYQ@mail.gmail.com>
Date:	Wed, 18 Feb 2015 20:39:17 +0530
From:	Ayyappa Ch <ayyappa.ch.linux@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Clarification needed regarding memory barrier

Hello All,

I am reading memory-barrier.txt file as mentioned below.

Please clarify my doubt .
 1) For example if CPU1 got the lock , How PCI bridge can see STORE
*ADDR = 4  before STORE *DATA = 1?

ACQUIRES VS I/O ACCESSES
------------------------

Under certain circumstances (especially involving NUMA), I/O accesses within
two spinlocked sections on two different CPUs may be seen as interleaved by the
PCI bridge, because the PCI bridge does not necessarily participate in the
cache-coherence protocol, and is therefore incapable of issuing the required
read memory barriers.

For example:

CPU 1
===============================
spin_lock(Q)
writel(0, ADDR)
writel(1, DATA);
spin_unlock(Q);

CPU 2
===============================
spin_lock(Q);
writel(4, ADDR);
writel(5, DATA);
spin_unlock(Q);

may be seen by the PCI bridge as follows:

STORE *ADDR = 0, STORE *ADDR = 4, STORE *DATA = 1, STORE *DATA = 5

which would probably cause the hardware to malfunction.

Thanks and regards,
Ayyappa.Ch
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