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SubjectRe: [patch] generic-ipi: fix the smp_mb() placement

* Suresh Siddha <suresh.b.siddha@intel.com> wrote:

> Subject: generic-ipi: fix the smp_mb() placement
>
> smp_mb() is needed (to make the memory operations visible globally)
> before sending the ipi on the sender and the receiver (on Alpha
> atleast) needs smp_read_barrier_depends() in the handler before
> reading the call_single_queue list in a lock-free fashion.
>
> On x86, x2apic mode register accesses for sending IPI's don't have
> serializing semantics. So the need for smp_mb() before sending the
> IPI becomes more critical in x2apic mode.
>
> Remove the unnecessary smp_mb() in csd_flag_wait(), as the presence
> of that smp_mb() doesn't mean anything on the sender, when the ipi
> receiver is not doing any thing special (like memory fence) after
> clearing the CSD_FLAG_WAIT.

nice! Did you see an actual lockup due to this? Seems like a v2.6.28
fix to me in any case.

Ingo


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