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Commit e3ebfb96 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Paul E. McKenney
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rcu: Add PROVE_RCU_DELAY to provoke difficult races 



There have been some recent bugs that were triggered only when
preemptible RCU's __rcu_read_unlock() was preempted just after setting
->rcu_read_lock_nesting to INT_MIN, which is a low-probability event.
Therefore, reproducing those bugs (to say nothing of gaining confidence
in alleged fixes) was quite difficult.  This commit therefore creates
a new debug-only RCU kernel config option that forces a short delay
in __rcu_read_unlock() to increase the probability of those sorts of
bugs occurring.

Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent a10d206e

kernel/rcupdate.c

+4 −0
Original line number Diff line number Diff line
@@ -45,6 +45,7 @@ 
#include <linux/mutex.h>

#include <linux/export.h>

#include <linux/hardirq.h>

#include <linux/delay.h>



#define CREATE_TRACE_POINTS

#include <trace/events/rcu.h>
@@ -81,6 +82,9 @@ void __rcu_read_unlock(void) 
	} else {

		barrier();  /* critical section before exit code. */

		t->rcu_read_lock_nesting = INT_MIN;

#ifdef CONFIG_PROVE_RCU_DELAY

		udelay(10); /* Make preemption more probable. */

#endif /* #ifdef CONFIG_PROVE_RCU_DELAY */

		barrier();  /* assign before ->rcu_read_unlock_special load */

		if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))

			rcu_read_unlock_special(t);

lib/Kconfig.debug

+14 −0
Original line number Diff line number Diff line
@@ -629,6 +629,20 @@ config PROVE_RCU_REPEATEDLY 


	 Say N if you are unsure.



config PROVE_RCU_DELAY

	bool "RCU debugging: preemptible RCU race provocation"

	depends on DEBUG_KERNEL && PREEMPT_RCU

	default n

	help

	 There is a class of races that involve an unlikely preemption

	 of __rcu_read_unlock() just after ->rcu_read_lock_nesting has

	 been set to INT_MIN.  This feature inserts a delay at that

	 point to increase the probability of these races.



	 Say Y to increase probability of preemption of __rcu_read_unlock().



	 Say N if you are unsure.



config SPARSE_RCU_POINTER

	bool "RCU debugging: sparse-based checks for pointer usage"

	default n