Commit 3d29aaf1 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Provide optional RCU-reader exit delay for strict GPs 



The goal of this series is to increase the probability of tools like
KASAN detecting that an RCU-protected pointer was used outside of its
RCU read-side critical section.  Thus far, the approach has been to make
grace periods and callback processing happen faster.  Another approach
is to delay the pointer leaker.  This commit therefore allows a delay
to be applied to exit from RCU read-side critical sections.

This slowdown is specified by a new rcutree.rcu_unlock_delay kernel boot
parameter that specifies this delay in microseconds, defaulting to zero.

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 4e025f52

Documentation/admin-guide/kernel-parameters.txt

+9 −0
Original line number Diff line number Diff line
@@ -4152,6 +4152,15 @@ 
			This wake_up() will be accompanied by a

			WARN_ONCE() splat and an ftrace_dump().



	rcutree.rcu_unlock_delay= [KNL]

			In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels,

			this specifies an rcu_read_unlock()-time delay

			in microseconds.  This defaults to zero.

			Larger delays increase the probability of

			catching RCU pointer leaks, that is, buggy use

			of RCU-protected pointers after the relevant

			rcu_read_unlock() has completed.



	rcutree.sysrq_rcu= [KNL]

			Commandeer a sysrq key to dump out Tree RCU's

			rcu_node tree with an eye towards determining

kernel/rcu/tree_plugin.h

+10 −2
Original line number Diff line number Diff line
@@ -430,6 +430,12 @@ static bool rcu_preempt_has_tasks(struct rcu_node *rnp) 
	return !list_empty(&rnp->blkd_tasks);

}



// Add delay to rcu_read_unlock() for strict grace periods.

static int rcu_unlock_delay;

#ifdef CONFIG_RCU_STRICT_GRACE_PERIOD

module_param(rcu_unlock_delay, int, 0444);

#endif



/*

 * Report deferred quiescent states.  The deferral time can

 * be quite short, for example, in the case of the call from
@@ -460,11 +466,13 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) 
	}

	t->rcu_read_unlock_special.s = 0;

	if (special.b.need_qs) {

		if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))

		if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {

			rcu_report_qs_rdp(rdp->cpu, rdp);

		else

			udelay(rcu_unlock_delay);

		} else {

			rcu_qs();

		}

	}



	/*

	 * Respond to a request by an expedited grace period for a

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