locking/locktorture: Fix deboosting NULL pointer dereference

For the case of rtmutex torturing we will randomly call into the
boost() handler, including upon module exiting when the tasks are
deboosted before stopping. In such cases the task may or may not have
already been boosted, and therefore the NULL being explicitly passed
can occur anywhere. Currently we only assume that the task will is
at a higher prio, and in consequence, dereference a NULL pointer.

This patch fixes the case of a rmmod locktorture exploding while
pounding on the rtmutex lock (partial trace):

 task: ffff88081026cf80 ti: ffff880816120000 task.ti: ffff880816120000
 RSP: 0018:ffff880816123eb0  EFLAGS: 00010206
 RAX: ffff88081026cf80 RBX: ffff880816bfa630 RCX: 0000000000160d1b
 RDX: 0000000000000000 RSI: 0000000000000202 RDI: 0000000000000000
 RBP: ffff88081026cf80 R08: 000000000000001f R09: ffff88017c20ca80
 R10: 0000000000000000 R11: 000000000048c316 R12: ffffffffa05d1840
 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 FS:  0000000000000000(0000) GS:ffff88203f880000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000008 CR3: 0000000001c0a000 CR4: 00000000000406e0
 Stack:
  ffffffffa05d141d ffff880816bfa630 ffffffffa05d1922 ffff88081e70c2c0
  ffff880816bfa630 ffffffff81095fed 0000000000000000 ffffffff8107bf60
  ffff880816bfa630 ffffffff00000000 ffff880800000000 ffff880816123f08
 Call Trace:
  [<ffffffff81095fed>] kthread+0xbd/0xe0
  [<ffffffff815cf40f>] ret_from_fork+0x3f/0x70

This patch ensures that if the random state pointer is not NULL and current
is not boosted, then do nothing.

 RIP: 0010:[<ffffffffa05c6185>]  [<ffffffffa05c6185>] torture_random+0x5/0x60 [torture]
  [<ffffffffa05d141d>] torture_rtmutex_boost+0x1d/0x90 [locktorture]
  [<ffffffffa05d1922>] lock_torture_writer+0xe2/0x170 [locktorture]

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bobby.prani@gmail.com
Cc: dhowells@redhat.com
Cc: dipankar@in.ibm.com
Cc: dvhart@linux.intel.com
Cc: edumazet@google.com
Cc: fweisbec@gmail.com
Cc: jiangshanlai@gmail.com
Cc: josh@joshtriplett.org
Cc: mathieu.desnoyers@efficios.com
Cc: oleg@redhat.com
Cc: rostedt@goodmis.org
Link: http://lkml.kernel.org/r/1460476038-27060-1-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/locktorture.c

@@ -394,12 +394,12 @@ static void torture_rtmutex_boost(struct torture_random_state *trsp)
 
 	if (!rt_task(current)) {
 		/*
-		 * (1) Boost priority once every ~50k operations. When the
+		 * Boost priority once every ~50k operations. When the
 		 * task tries to take the lock, the rtmutex it will account
 		 * for the new priority, and do any corresponding pi-dance.
 		 */
-		if (!(torture_random(trsp) %
-		      (cxt.nrealwriters_stress * factor))) {
+		if (trsp && !(torture_random(trsp) %
+			      (cxt.nrealwriters_stress * factor))) {
 			policy = SCHED_FIFO;
 			param.sched_priority = MAX_RT_PRIO - 1;
 		} else /* common case, do nothing */