On Thu, 3 Aug 2017 16:28:20 +0100 Will Deacon <[email protected]> wrote:
> On Wed, Aug 02, 2017 at 01:38:40PM +0200, Peter Zijlstra wrote: > > Since its inception, our understanding of ACQUIRE, esp. as applied to > > spinlocks, has changed somewhat. Also, I wonder if, with a simple > > change, we cannot make it provide more. > > > > The problem with the comment is that the STORE done by spin_lock isn't > > itself ordered by the ACQUIRE, and therefore a later LOAD can pass over > > it and cross with any prior STORE, rendering the default WMB > > insufficient (pointed out by Alan). > > > > Now, this is only really a problem on PowerPC and ARM64, both of > > which already defined smp_mb__before_spinlock() as a smp_mb(). > > > > At the same time, we can get a much stronger construct if we place > > that same barrier _inside_ the spin_lock(). In that case we upgrade > > the RCpc spinlock to an RCsc. That would make all schedule() calls > > fully transitive against one another. > > > > Cc: Alan Stern <[email protected]> > > Cc: Nicholas Piggin <[email protected]> > > Cc: Ingo Molnar <[email protected]> > > Cc: Will Deacon <[email protected]> > > Cc: Linus Torvalds <[email protected]> > > Cc: Michael Ellerman <[email protected]> > > Cc: Oleg Nesterov <[email protected]> > > Cc: Benjamin Herrenschmidt <[email protected]> > > Cc: Paul McKenney <[email protected]> > > Signed-off-by: Peter Zijlstra (Intel) <[email protected]> > > --- > > arch/arm64/include/asm/spinlock.h | 2 ++ > > arch/powerpc/include/asm/spinlock.h | 3 +++ > > include/linux/atomic.h | 3 +++ > > include/linux/spinlock.h | 36 > > ++++++++++++++++++++++++++++++++++++ > > kernel/sched/core.c | 4 ++-- > > 5 files changed, 46 insertions(+), 2 deletions(-) > > > > --- a/arch/arm64/include/asm/spinlock.h > > +++ b/arch/arm64/include/asm/spinlock.h > > @@ -367,5 +367,7 @@ static inline int arch_read_trylock(arch > > * smp_mb__before_spinlock() can restore the required ordering. > > */ > > #define smp_mb__before_spinlock() smp_mb() > > +/* See include/linux/spinlock.h */ > > +#define smp_mb__after_spinlock() smp_mb() > > > > #endif /* __ASM_SPINLOCK_H */ > > Acked-by: Will Deacon <[email protected]> Yeah this looks good to me. I don't think there would ever be a reason to use smp_mb__before_spinlock() rather than smp_mb__after_spinlock().
