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rcu/nocb: Avoid confusing double write of rdp->nocb_cb_sleep

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The nocb_cb_wait() function first sets the rdp->nocb_cb_sleep flag to
true by after invoking the callbacks, and then sets it back to false if
it finds more callbacks that are ready to invoke.

This is confusing and will become unsafe if this flag is ever read
locklessly.  This commit therefore writes it only once, based on the
state after both callback invocation and checking.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2021-01-28 18:12:12 +01:00 committed by Paul E. McKenney
parent 64305db285
commit 8a682b3974
1 changed files with 4 additions and 3 deletions
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7
kernel/rcu/tree_plugin.h
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@ -2230,6 +2230,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
unsigned long flags;
bool needwake_state = false;
bool needwake_gp = false;
bool can_sleep = true;
struct rcu_node *rnp = rdp->mynode;
local_irq_save(flags);
@ -2253,8 +2254,6 @@ static void nocb_cb_wait(struct rcu_data *rdp)
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
}
WRITE_ONCE(rdp->nocb_cb_sleep, true);
if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)) {
rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_CB);
@ -2262,7 +2261,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
needwake_state = true;
}
if (rcu_segcblist_ready_cbs(cblist))
WRITE_ONCE(rdp->nocb_cb_sleep, false);
can_sleep = false;
} else {
/*
* De-offloading. Clear our flag and notify the de-offload worker.
@ -2275,6 +2274,8 @@ static void nocb_cb_wait(struct rcu_data *rdp)
needwake_state = true;
}
WRITE_ONCE(rdp->nocb_cb_sleep, can_sleep);
if (rdp->nocb_cb_sleep)
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));