From c6f5d02b6a0fb91be5d656885ce02cf28952181d Mon Sep 17 00:00:00 2001 From: Andrea Parri Date: Mon, 14 May 2018 16:01:28 -0700 Subject: [PATCH] locking/spinlocks/arm64: Remove smp_mb() from arch_spin_is_locked() The following 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 that spin_is_locked() is not required to provide such an ordering guarantee (a guarantee that is currently not provided by all the implementations/archs), and that callers relying 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 callers of {,raw_}spin_is_locked(), revealing that none of them are relying on the ordering guarantee anymore, this commit removes the leading smp_mb() from the primitive thus reverting 38b850a73034f. [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 https://marc.info/?l=linux-kernel&m=152042843808540&w=2 https://marc.info/?l=linux-kernel&m=152043346110262&w=2 Signed-off-by: Andrea Parri Signed-off-by: Paul E. McKenney Acked-by: Will Deacon Cc: Andrew Morton Cc: Catalin Marinas Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: akiyks@gmail.com Cc: boqun.feng@gmail.com Cc: dhowells@redhat.com Cc: j.alglave@ucl.ac.uk Cc: linux-arch@vger.kernel.org Cc: luc.maranget@inria.fr Cc: npiggin@gmail.com Cc: parri.andrea@gmail.com Cc: stern@rowland.harvard.edu Link: http://lkml.kernel.org/r/1526338889-7003-2-git-send-email-paulmck@linux.vnet.ibm.com Signed-off-by: Ingo Molnar --- arch/arm64/include/asm/spinlock.h | 5 ----- 1 file changed, 5 deletions(-) diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h index ebdae15d665d..26c5bd7d88d8 100644 --- a/arch/arm64/include/asm/spinlock.h +++ b/arch/arm64/include/asm/spinlock.h @@ -122,11 +122,6 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock) static inline int arch_spin_is_locked(arch_spinlock_t *lock) { - /* - * Ensure prior spin_lock operations to other locks have completed - * on this CPU before we test whether "lock" is locked. - */ - smp_mb(); /* ^^^ */ return !arch_spin_value_unlocked(READ_ONCE(*lock)); } -- 2.20.1