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rcu: Optimize and protect atomic_cmpxchg() loop

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This commit reworks the atomic_cmpxchg() loop in rcu_eqs_special_set()
to do only the initial read from the current CPU's rcu_data structure's
->dynticks field explicitly.  On subsequent passes, this value is instead
retained from the failing atomic_cmpxchg() operation.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2020-02-03 14:20:00 -08:00
parent 92c0b889f2
commit faa059c397
1 changed files with 5 additions and 2 deletions
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7
kernel/rcu/tree.c
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@ -342,14 +342,17 @@ bool rcu_eqs_special_set(int cpu)
{
int old;
int new;
int new_old;
struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
new_old = atomic_read(&rdp->dynticks);
do {
old = atomic_read(&rdp->dynticks);
old = new_old;
if (old & RCU_DYNTICK_CTRL_CTR)
return false;
new = old | RCU_DYNTICK_CTRL_MASK;
} while (atomic_cmpxchg(&rdp->dynticks, old, new) != old);
new_old = atomic_cmpxchg(&rdp->dynticks, old, new);
} while (new_old != old);
return true;
}