Re: [PATCH for-4.17 1/2] arm64: Remove smp_mb() from arch_spin_is_locked()

[Andrea Parri]

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.


> 
> Also -- does this mean we can kill the #ifndef queued_spin_is_locked
> guards in asm-generic/qspinlock.h?

I don't see why arch may want to override that definition (maybe lack of
imagination?); please let me know if you'd like to see the #ifndef gone.

  Andrea

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