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Commit c6ebcbb6 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Paul E. McKenney
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rcu: Fix bug in rcu_barrier() torture test 



The child threads in the rcu_torture_barrier_cbs() are improperly
synchronized, which can cause the rcu_barrier() tests to hang.  The
failure mode is as follows:

1.	CPU 0 running in rcu_torture_barrier() sets barrier_cbs_count
    	to n_barrier_cbs.

2.	CPU 1 running in rcu_torture_barrier_cbs() wakes up, posts
    	its RCU callback, and atomically decrements barrier_cbs_count.
    	Because barrier_cbs_count is not zero, it does not do the wake_up().

3.	CPU 2 running in rcu_torture_barrier_cbs() wakes up, but
    	finds that barrier_cbs_count is not equal to n_barrier_cbs,
    	and so returns to sleep.

4.	The value of barrier_cbs_count therefore never reaches zero,
    	which causes the test to hang.

This commit therefore uses a phase variable to coordinate the test,
preventing this scenario from occurring.

Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent e3f8d378

kernel/rcutorture.c

+7 −2
Original line number Diff line number Diff line
@@ -206,6 +206,7 @@ static unsigned long boost_starttime; /* jiffies of next boost test start. */ 
DEFINE_MUTEX(boost_mutex);		/* protect setting boost_starttime */

					/*  and boost task create/destroy. */

static atomic_t barrier_cbs_count;	/* Barrier callbacks registered. */

static bool barrier_phase;		/* Test phase. */

static atomic_t barrier_cbs_invoked;	/* Barrier callbacks invoked. */

static wait_queue_head_t *barrier_cbs_wq; /* Coordinate barrier testing. */

static DECLARE_WAIT_QUEUE_HEAD(barrier_wq);
@@ -1642,6 +1643,7 @@ void rcu_torture_barrier_cbf(struct rcu_head *rcu) 
static int rcu_torture_barrier_cbs(void *arg)

{

	long myid = (long)arg;

	bool lastphase = 0;

	struct rcu_head rcu;



	init_rcu_head_on_stack(&rcu);
@@ -1649,9 +1651,11 @@ static int rcu_torture_barrier_cbs(void *arg) 
	set_user_nice(current, 19);

	do {

		wait_event(barrier_cbs_wq[myid],

			   atomic_read(&barrier_cbs_count) == n_barrier_cbs ||

			   barrier_phase != lastphase ||

			   kthread_should_stop() ||

			   fullstop != FULLSTOP_DONTSTOP);

		lastphase = barrier_phase;

		smp_mb(); /* ensure barrier_phase load before ->call(). */

		if (kthread_should_stop() || fullstop != FULLSTOP_DONTSTOP)

			break;

		cur_ops->call(&rcu, rcu_torture_barrier_cbf);
@@ -1676,7 +1680,8 @@ static int rcu_torture_barrier(void *arg) 
	do {

		atomic_set(&barrier_cbs_invoked, 0);

		atomic_set(&barrier_cbs_count, n_barrier_cbs);

		/* wake_up() path contains the required barriers. */

		smp_mb(); /* Ensure barrier_phase after prior assignments. */

		barrier_phase = !barrier_phase;

		for (i = 0; i < n_barrier_cbs; i++)

			wake_up(&barrier_cbs_wq[i]);

		wait_event(barrier_wq,