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rcu: Remove full memory barrier on RCU stall printout

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RCU stall printout fetches the EQS state of a CPU with a preceding full
memory barrier. However there is nothing to order this read against at
this debugging stage. It is inherently racy when performed remotely.

Do a plain read instead.

This was the last user of rcu_dynticks_snap().

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 14:53:31 +02:00 committed by Paul E. McKenney
parent e7a3c8ea6e
commit 55911a9f42
2 changed files with 2 additions and 12 deletions
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10
kernel/rcu/tree.c
View File

@ -295,16 +295,6 @@ static void rcu_dynticks_eqs_online(void)
ct_state_inc(RCU_DYNTICKS_IDX);
}
/*
* Snapshot the ->dynticks counter with full ordering so as to allow
* stable comparison of this counter with past and future snapshots.
*/
static int rcu_dynticks_snap(int cpu)
{
smp_mb(); // Fundamental RCU ordering guarantee.
return ct_dynticks_cpu_acquire(cpu);
}
/*
* Return true if the snapshot returned from rcu_dynticks_snap()
* indicates that RCU is in an extended quiescent state.

4
kernel/rcu/tree_stall.h
View File

@ -501,7 +501,7 @@ static void print_cpu_stall_info(int cpu)
}
delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
falsepositive = rcu_is_gp_kthread_starving(NULL) &&
rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
rcu_dynticks_in_eqs(ct_dynticks_cpu(cpu));
rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
if (rcuc_starved)
// Print signed value, as negative values indicate a probable bug.
@ -515,7 +515,7 @@ static void print_cpu_stall_info(int cpu)
rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
"!."[!delta],
ticks_value, ticks_title,
rcu_dynticks_snap(cpu) & 0xffff,
ct_dynticks_cpu(cpu) & 0xffff,
ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,