Re: [RESEND PATCH] documentation: memory-barriers: fix smp_mb__before_spinlock() semantics
From: Paul E. McKenney <hidden>
Date: 2015-04-01 15:26:13
Also in:
lkml
Subsystem:
linux for powerpc (32-bit and 64-bit), the rest · Maintainers:
Madhavan Srinivasan, Linus Torvalds
On Tue, Mar 31, 2015 at 09:39:41AM +0100, Will Deacon wrote:
Our current documentation claims that, when followed by an ACQUIRE, smp_mb__before_spinlock() orders prior loads against subsequent loads and stores, which isn't actually true. Fix the documentation to state that this sequence orders only prior stores against subsequent loads and stores. Cc: Oleg Nesterov <oleg@redhat.com> Cc: "Paul E. McKenney" <redacted> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Will Deacon <redacted> --- Could somebody pick this up please? I guess I could route it via the arm64 tree with an Ack, but I'd rather it went through Paul or -tip.
Queued for 4.2, along with a separate patch for PowerPC that make it so that PowerPC actually behaves as described below. ;-) Thanx, Paul
quoted hunk
Documentation/memory-barriers.txt | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-)diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index ca2387ef27ab..fa28a0c1e2b1 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt@@ -1768,10 +1768,9 @@ for each construct. These operations all imply certain barriers: Memory operations issued before the ACQUIRE may be completed after the ACQUIRE operation has completed. An smp_mb__before_spinlock(), - combined with a following ACQUIRE, orders prior loads against - subsequent loads and stores and also orders prior stores against - subsequent stores. Note that this is weaker than smp_mb()! The - smp_mb__before_spinlock() primitive is free on many architectures. + combined with a following ACQUIRE, orders prior stores against + subsequent loads and stores. Note that this is weaker than smp_mb()! + The smp_mb__before_spinlock() primitive is free on many architectures. (2) RELEASE operation implication:
------------------------------------------------------------------------
powerpc: Fix smp_mb__before_spinlock()
Currently, smp_mb__before_spinlock() is defined to be smp_wmb()
in core code, but this is not sufficient on PowerPC. This patch
therefore supplies an override for the generic definition to
strengthen smp_mb__before_spinlock() to smp_mb(), as is needed
on PowerPC.
Signed-off-by: Paul E. McKenney [off-list ref]
Cc: [off-list ref]
diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
index a3bf5be111ff..1124f59b8df4 100644
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h@@ -89,5 +89,6 @@ do { \ #define smp_mb__before_atomic() smp_mb() #define smp_mb__after_atomic() smp_mb() +#define smp_mb__before_spinlock() smp_mb() #endif /* _ASM_POWERPC_BARRIER_H */