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rcu/exp: Remove redundant full memory barrier at the end of GP

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A full memory barrier is necessary at the end of the expedited grace
period to order:

1) The grace period completion (pictured by the GP sequence
   number) with all preceding accesses. This pairs with rcu_seq_end()
   performed by the concurrent kworker.

2) The grace period completion and subsequent post-GP update side
   accesses. Pairs again against rcu_seq_end().

This full barrier is already provided by the final sync_exp_work_done()
test, making the subsequent explicit one redundant. Remove it and
improve comments.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
This commit is contained in:
Frederic Weisbecker 2024-05-15 14:53:32 +02:00 committed by Paul E. McKenney
parent 55911a9f42
commit 677ab23bdf
1 changed files with 6 additions and 2 deletions
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8
kernel/rcu/tree_exp.h
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@ -265,7 +265,12 @@ static bool sync_exp_work_done(unsigned long s)
{
if (rcu_exp_gp_seq_done(s)) {
trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
smp_mb(); /* Ensure test happens before caller kfree(). */
/*
* Order GP completion with preceding accesses. Order also GP
* completion with post GP update side accesses. Pairs with
* rcu_seq_end().
*/
smp_mb();
return true;
}
return false;
@ -967,7 +972,6 @@ void synchronize_rcu_expedited(void)
rnp = rcu_get_root();
wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
sync_exp_work_done(s));
smp_mb(); /* Work actions happen before return. */
/* Let the next expedited grace period start. */
mutex_unlock(&rcu_state.exp_mutex);