documentation: memory-barriers: Fix smp_mb__before_spinlock() semantics (d956028e) · Commits · 戴 / test

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 the intent. This commit therefore fixes the documentation to state that this sequence orders only prior stores against subsequent loads and stores. In addition, the original intent of smp_mb__before_spinlock() was to only order prior loads against subsequent stores, however, people have started using it as if it ordered prior loads against subsequent loads and stores. This commit therefore also updates smp_mb__before_spinlock()'s header comment to reflect this new reality. Cc: Oleg Nesterov <oleg@redhat.com> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by:

Will Deacon <will.deacon@arm.com> Signed-off-by:

Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/memory-barriers.txt

+3 −4

Original line number	Diff line number	Diff line
		@@ -1784,10 +1784,9 @@ 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:

include/linux/spinlock.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -120,7 +120,7 @@ do { \
		/*
		* Despite its name it doesn't necessarily has to be a full barrier.
		* It should only guarantee that a STORE before the critical section
		* can not be reordered with a LOAD inside this section.
		* can not be reordered with LOADs and STOREs inside this section.
		* spin_lock() is the one-way barrier, this LOAD can not escape out
		* of the region. So the default implementation simply ensures that
		* a STORE can not move into the critical section, smp_wmb() should

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