From 77a8b8462b02a10aea5cad389a8f9260f79ede36 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Fri, 16 Feb 2018 09:23:31 +0100 Subject: rcu: make memory barriers more explicit Prepare for introducing smp_mb_placeholder() and smp_mb_global(). The new smp_mb() in synchronize_rcu() is not strictly necessary, since the first atomic_mb_set for rcu_gp_ctr provides the required ordering. However, synchronize_rcu is not performance critical, and it *will* be necessary to introduce a smp_mb_global before calling wait_for_readers(). Signed-off-by: Paolo Bonzini --- include/qemu/rcu.h | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) (limited to 'include/qemu/rcu.h') diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index f19413d649..625f09ac09 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -79,7 +79,10 @@ static inline void rcu_read_lock(void) } ctr = atomic_read(&rcu_gp_ctr); - atomic_xchg(&p_rcu_reader->ctr, ctr); + atomic_set(&p_rcu_reader->ctr, ctr); + + /* Write p_rcu_reader->ctr before reading RCU-protected pointers. */ + smp_mb(); } static inline void rcu_read_unlock(void) @@ -91,7 +94,15 @@ static inline void rcu_read_unlock(void) return; } - atomic_xchg(&p_rcu_reader->ctr, 0); + /* Ensure that the critical section is seen to precede the + * store to p_rcu_reader->ctr. Together with the following + * smp_mb(), this ensures writes to p_rcu_reader->ctr + * are sequentially consistent. + */ + atomic_store_release(&p_rcu_reader->ctr, 0); + + /* Write p_rcu_reader->ctr before reading p_rcu_reader->waiting. */ + smp_mb(); if (unlikely(atomic_read(&p_rcu_reader->waiting))) { atomic_set(&p_rcu_reader->waiting, false); qemu_event_set(&rcu_gp_event); -- cgit 1.4.1