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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>
This commit is contained in:
Lai Jiangshan 2012-11-29 16:46:09 +08:00 committed by Paul E. McKenney
parent 49271ca606
commit 7a6b55e710
1 changed files with 1 additions and 2 deletions
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3
kernel/srcu.c
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@ -298,9 +298,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
{
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;