Commit 51d7d5205d338 ("powerpc: Add smp_mb() to arch_spin_is_locked()") added an smp_mb() to arch_spin_is_locked(), in order to ensure that
Thread 0 Thread 1 spin_lock(A); spin_lock(B); r0 = spin_is_locked(B) r1 = spin_is_locked(A); never ends up with r0 = r1 = 0, and reported one example (in ipc/sem.c) relying on such guarantee. It's however understood (and undocumented) that spin_is_locked() is not required to ensure such ordering guarantee, guarantee that is currently _not_ provided by all implementations/arch, and that callers relying on such ordering should instead use suitable memory barriers before acting on the result of spin_is_locked(). Following a recent auditing[1] of the callers of {,raw_}spin_is_locked() revealing that none of them are relying on this guarantee anymore, this commit removes the leading smp_mb() from the primitive thus effectively reverting 51d7d5205d338. [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 Signed-off-by: Andrea Parri <andrea.pa...@amarulasolutions.com> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org> Cc: Paul Mackerras <pau...@samba.org> Cc: Michael Ellerman <m...@ellerman.id.au> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Ingo Molnar <mi...@redhat.com> Cc: Linus Torvalds <torva...@linux-foundation.org> --- arch/powerpc/include/asm/spinlock.h | 1 - 1 file changed, 1 deletion(-) diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h index b9ebc3085fb79..ecc141e3f1a73 100644 --- a/arch/powerpc/include/asm/spinlock.h +++ b/arch/powerpc/include/asm/spinlock.h @@ -67,7 +67,6 @@ static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock) static inline int arch_spin_is_locked(arch_spinlock_t *lock) { - smp_mb(); return !arch_spin_value_unlocked(*lock); } -- 2.7.4