Clarification needed regarding memory barrier

[Ayyappa Ch <ayyappa.ch.linux@gmail.com>]

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