srcu: use ACCESS_ONCE() to access sp->completed in srcu_read_lock()
The old SRCU implementation loads sp->completed within an
RCU-sched section, courtesy of preempt_disable(). This was required
due to the use of synchronize_sched() in the old implemenation's
synchronize_srcu(). However, the new implementation does not rely
on synchronize_sched(), so it in turn does not require the load of
sp->completed and the ->c[] counter to be in a single preempt-disabled
region of code. This commit therefore moves the sp->completed access
outside of the preempt-disabled region and applies ACCESS_ONCE().
The resulting code is almost as the same as before, but it removes the
now-misleading rcu_dereference_index_check() call.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/srcu.c b/kernel/srcu.c
index e34f299..01d5ccb 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -298,9 +298,8 @@
{
int idx;
+ idx = ACCESS_ONCE(sp->completed) & 0x1;
preempt_disable();
- idx = rcu_dereference_index_check(sp->completed,
- rcu_read_lock_sched_held()) & 0x1;
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;