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rcu: Don't use negative nesting depth in __rcu_read_unlock()

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Now that RCU flavors have been consolidated, an RCU-preempt
rcu_read_unlock() in an interrupt or softirq handler cannot possibly
end the RCU read-side critical section.  Consider the old vulnerability
involving rcu_read_unlock() being invoked within such a handler that
interrupted an __rcu_read_unlock_special(), in which a wakeup might be
invoked with a scheduler lock held.  Because rcu_read_unlock_special()
no longer does wakeups in such situations, it is no longer necessary
for __rcu_read_unlock() to set the nesting level negative.

This commit therefore removes this recursion-protection code from
__rcu_read_unlock().

[ paulmck: Let rcu_exp_handler() continue to call rcu_report_exp_rdp(). ]
[ paulmck: Adjust other checks given no more negative nesting. ]
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Lai Jiangshan 2020-02-15 15:23:26 -08:00 • committed by Paul E. McKenney
parent f0bdf6d473
commit 5f5fa7ea89
2 changed files with 12 additions and 41 deletions
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22
kernel/rcu/tree_plugin.h
View file

@ -345,9 +345,7 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
return READ_ONCE(rnp->gp_tasks) != NULL;
}
/* Bias and limit values for ->rcu_read_lock_nesting. */
#define RCU_NEST_BIAS INT_MAX
#define RCU_NEST_NMAX (-INT_MAX / 2)
/* limit value for ->rcu_read_lock_nesting. */
#define RCU_NEST_PMAX (INT_MAX / 2)
static void rcu_preempt_read_enter(void)
@ -355,9 +353,9 @@ static void rcu_preempt_read_enter(void)
current->rcu_read_lock_nesting++;
}
static void rcu_preempt_read_exit(void)
static int rcu_preempt_read_exit(void)
{
current->rcu_read_lock_nesting--;
return --current->rcu_read_lock_nesting;
}
static void rcu_preempt_depth_set(int val)
@ -390,21 +388,15 @@ void __rcu_read_unlock(void)
{
struct task_struct *t = current;
if (rcu_preempt_depth() != 1) {
rcu_preempt_read_exit();
} else {
if (rcu_preempt_read_exit() == 0) {
barrier(); /* critical section before exit code. */
rcu_preempt_depth_set(-RCU_NEST_BIAS);
barrier(); /* assign before ->rcu_read_unlock_special load */
if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
rcu_read_unlock_special(t);
barrier(); /* ->rcu_read_unlock_special load before assign */
rcu_preempt_depth_set(0);
}
if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
int rrln = rcu_preempt_depth();
WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
WARN_ON_ONCE(rrln < 0 || rrln > RCU_NEST_PMAX);
}
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);
@ -556,7 +548,7 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
READ_ONCE(t->rcu_read_unlock_special.s)) &&
rcu_preempt_depth() <= 0;
rcu_preempt_depth() == 0;
}
/*
@ -692,7 +684,7 @@ static void rcu_flavor_sched_clock_irq(int user)
} else if (rcu_preempt_need_deferred_qs(t)) {
rcu_preempt_deferred_qs(t); /* Report deferred QS. */
return;
} else if (!rcu_preempt_depth()) {
} else if (!WARN_ON_ONCE(rcu_preempt_depth())) {
rcu_qs(); /* Report immediate QS. */
return;
}