On 04/01, Paul E. McKenney wrote:
>
> If Will agrees, like the following?
Looks good to me, thanks ;)
> documentation: memory-barriers: Fix smp_mb__before_spinlock() semantics
>
> 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 <o...@redhat.com>
> Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Signed-off-by: Will Deacon <will.dea...@arm.com>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> diff --git a/Documentation/memory-barriers.txt
> b/Documentation/memory-barriers.txt
> index 6974f1c2b4e1..52c320e3f107 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -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:
>
> diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> index 3e18379dfa6f..0063b24b4f36 100644
> --- a/include/linux/spinlock.h
> +++ b/include/linux/spinlock.h
> @@ -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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