Commit-ID: 7f56b58a92aaf2cab049f32a19af7cc57a3972f2 Gitweb: https://git.kernel.org/tip/7f56b58a92aaf2cab049f32a19af7cc57a3972f2 Author: Jason Low <jason.l...@hp.com> AuthorDate: Thu, 26 Apr 2018 11:34:22 +0100 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Fri, 27 Apr 2018 09:48:49 +0200
locking/mcs: Use smp_cond_load_acquire() in MCS spin loop For qspinlocks on ARM64, we would like to use WFE instead of purely spinning. Qspinlocks internally have lock contenders spin on an MCS lock. Update arch_mcs_spin_lock_contended() such that it uses the new smp_cond_load_acquire() so that ARM64 can also override this spin loop with its own implementation using WFE. On x86, this can also be cheaper than spinning on smp_load_acquire(). Signed-off-by: Jason Low <jason.l...@hp.com> Signed-off-by: Will Deacon <will.dea...@arm.com> Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org> Acked-by: Waiman Long <long...@redhat.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: boqun.f...@gmail.com Cc: linux-arm-ker...@lists.infradead.org Cc: paul...@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1524738868-31318-9-git-send-email-will.dea...@arm.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- kernel/locking/mcs_spinlock.h | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h index f046b7ce9dd6..5e10153b4d3c 100644 --- a/kernel/locking/mcs_spinlock.h +++ b/kernel/locking/mcs_spinlock.h @@ -23,13 +23,15 @@ struct mcs_spinlock { #ifndef arch_mcs_spin_lock_contended /* - * Using smp_load_acquire() provides a memory barrier that ensures - * subsequent operations happen after the lock is acquired. + * Using smp_cond_load_acquire() provides the acquire semantics + * required so that subsequent operations happen after the + * lock is acquired. Additionally, some architectures such as + * ARM64 would like to do spin-waiting instead of purely + * spinning, and smp_cond_load_acquire() provides that behavior. */ #define arch_mcs_spin_lock_contended(l) \ do { \ - while (!(smp_load_acquire(l))) \ - cpu_relax(); \ + smp_cond_load_acquire(l, VAL); \ } while (0) #endif
