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Date: Mon, 16 Apr 2018 09:12:33 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org Cc: mingo@...nel.org, stern@...land.harvard.edu, parri.andrea@...il.com, will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com, Andrea Parri <andrea.parri@...rulasolutions.com>, Catalin Marinas <catalin.marinas@....com>, Ingo Molnar <mingo@...hat.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: [PATCH tools/memory-model 05/17] arm64: Remove smp_mb() from arch_spin_is_locked() From: Andrea Parri <andrea.parri@...rulasolutions.com> 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 <andrea.parri@...rulasolutions.com> Acked-by: Will Deacon <will.deacon@....com> Cc: Catalin Marinas <catalin.marinas@....com> Cc: Will Deacon <will.deacon@....com> Cc: Ingo Molnar <mingo@...hat.com> Cc: Peter Zijlstra <peterz@...radead.org> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> --- 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)); } -- 2.5.2
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