Messages in this thread | | | Date | Tue, 27 Mar 2018 17:54:23 +0100 | From | Will Deacon <> | Subject | Re: [PATCH for-4.17 1/2] arm64: Remove smp_mb() from arch_spin_is_locked() |
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Hi Andrea,
On Mon, Mar 26, 2018 at 04:14:36PM +0200, Andrea Parri wrote: > On Mon, Mar 26, 2018 at 11:57:05AM +0100, Will Deacon wrote: > > On Mon, Mar 26, 2018 at 12:37:21PM +0200, Andrea Parri wrote: > > > Commit 38b850a73034f ("arm64: spinlock: order spin_{is_locked,unlock_wait} > > > against local locks") added an smp_mb() to arch_spin_is_locked(), in order > > > "to ensure that the lock value is always loaded after any other locks have > > > been taken by the current CPU", and reported one example (the "insane case" > > > in ipc/sem.c) relying on such guarantee. > > > > > > It is however understood (and not documented) that spin_is_locked() is not > > > required to ensure such an ordering guarantee, guarantee that is currently > > > _not_ provided by all implementations/architectures, and that callers rely- > > > ing on such ordering should instead insert suitable memory barriers before > > > acting on the result of spin_is_locked(). > > > > > > Following a recent auditing[1] of the callsites of {,raw_}spin_is_locked() > > > revealing that none of these callers are relying on the ordering guarantee > > > anymore, this commit removes the leading smp_mb() from this primitive thus > > > effectively reverting 38b850a73034f. > > > > > > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 > > > > What is patch 2/2 in this series? I couldn't find it in the archive. > > 2/2 is this change for powerpc: > > https://marc.info/?l=linux-kernel&m=152206068707522&w=2 > > > > > Assuming that patch doesn't do it, please can you remove the comment > > about spin_is_locked from mutex_is_locked? > > I ended up with the patch below but I suspect that it's not what you had > in mind; please let me know if you'd like me to add it into this series.
That's exactly what I meant! I think you can remove the #ifndef queued_spin_is_locked in the same patch. If you do that:
Acked-by: Will Deacon <will.deacon@arm.com>
for the arm64 patch and the mythical core patch we've been discussing.
Will
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