Commit b2c4623d authored Oct 22, 2014 by Paul E. McKenney

rcu: More on deadlock between CPU hotplug and expedited grace periods

Commit dd56af42 (rcu: Eliminate deadlock between CPU hotplug and
expedited grace periods) was incomplete. Although it did eliminate
deadlocks involving synchronize_sched_expedited()'s acquisition of
cpu_hotplug.lock via get_online_cpus(), it did nothing about the similar
deadlock involving acquisition of this same lock via put_online_cpus().
This deadlock became apparent with testing involving hibernation.

This commit therefore changes put_online_cpus() acquisition of this lock
to be conditional, and increments a new cpu_hotplug.puts_pending field
in case of acquisition failure. Then cpu_hotplug_begin() checks for this
new field being non-zero, and applies any changes to cpu_hotplug.refcount.

Reported-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Jiri Kosina <jkosina@suse.cz>
Tested-by: Borislav Petkov <bp@suse.de>

parent f114040e

kernel/cpu.c

+13 −1

Original line number	Diff line number	Diff line
		@@ -64,6 +64,8 @@ static struct {
		* an ongoing cpu hotplug operation.
		*/
		int refcount;
		/* And allows lockless put_online_cpus(). */
		atomic_t puts_pending;

		#ifdef CONFIG_DEBUG_LOCK_ALLOC
		struct lockdep_map dep_map;
		@@ -113,7 +115,11 @@ void put_online_cpus(void)
		{
		if (cpu_hotplug.active_writer == current)
		return;
		mutex_lock(&cpu_hotplug.lock);
		if (!mutex_trylock(&cpu_hotplug.lock)) {
		atomic_inc(&cpu_hotplug.puts_pending);
		cpuhp_lock_release();
		return;
		}

		if (WARN_ON(!cpu_hotplug.refcount))
		cpu_hotplug.refcount++; /* try to fix things up */
		@@ -155,6 +161,12 @@ void cpu_hotplug_begin(void)
		cpuhp_lock_acquire();
		for (;;) {
		mutex_lock(&cpu_hotplug.lock);
		if (atomic_read(&cpu_hotplug.puts_pending)) {
		int delta;

		delta = atomic_xchg(&cpu_hotplug.puts_pending, 0);
		cpu_hotplug.refcount -= delta;
		}
		if (likely(!cpu_hotplug.refcount))
		break;
		__set_current_state(TASK_UNINTERRUPTIBLE);

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