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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2013-09-24 15:04:06 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2013-12-03 10:10:18 -0800 |
| commit | 78e4bc34e5d966cfd95f1238565afc399d56225c (patch) | |
| tree | e430291c48ec41b22e31865bf5aa13c4db87a3cb /kernel/rcu/torture.c | |
| parent | 6193c76aba8ec3cc5f083c35efbab9ed924125f6 (diff) | |
| download | lwn-78e4bc34e5d966cfd95f1238565afc399d56225c.tar.gz lwn-78e4bc34e5d966cfd95f1238565afc399d56225c.zip | |
rcu: Fix and comment ordering around wait_event()
It is all too easy to forget that wait_event() does not necessarily
imply a full memory barrier. The case where it does not is where the
condition transitions to true just as wait_event() starts execution.
This is actually a feature: The standard use of wait_event() involves
locking, in which case the locks provide the needed ordering (you hold a
lock across the wake_up() and acquire that same lock after wait_event()
returns).
Given that I did forget that wait_event() does not necessarily imply a
full memory barrier in one case, this commit fixes that case. This commit
also adds comments calling out the placement of existing memory barriers
relied on by wait_event() calls.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'kernel/rcu/torture.c')
| -rw-r--r-- | kernel/rcu/torture.c | 8 |
1 files changed, 5 insertions, 3 deletions
diff --git a/kernel/rcu/torture.c b/kernel/rcu/torture.c index 3929cd451511..69a4ec80a788 100644 --- a/kernel/rcu/torture.c +++ b/kernel/rcu/torture.c @@ -1578,6 +1578,7 @@ static int rcu_torture_barrier_cbs(void *arg) { long myid = (long)arg; bool lastphase = 0; + bool newphase; struct rcu_head rcu; init_rcu_head_on_stack(&rcu); @@ -1585,10 +1586,11 @@ static int rcu_torture_barrier_cbs(void *arg) set_user_nice(current, 19); do { wait_event(barrier_cbs_wq[myid], - barrier_phase != lastphase || + (newphase = + ACCESS_ONCE(barrier_phase)) != lastphase || kthread_should_stop() || fullstop != FULLSTOP_DONTSTOP); - lastphase = barrier_phase; + lastphase = newphase; smp_mb(); /* ensure barrier_phase load before ->call(). */ if (kthread_should_stop() || fullstop != FULLSTOP_DONTSTOP) break; @@ -1625,7 +1627,7 @@ static int rcu_torture_barrier(void *arg) if (kthread_should_stop() || fullstop != FULLSTOP_DONTSTOP) break; n_barrier_attempts++; - cur_ops->cb_barrier(); + cur_ops->cb_barrier(); /* Implies smp_mb() for wait_event(). */ if (atomic_read(&barrier_cbs_invoked) != n_barrier_cbs) { n_rcu_torture_barrier_error++; WARN_ON_ONCE(1); |