From 04cafed7fc19a8010771c788708ac97c405fc3de Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 12 May 2016 13:57:45 +0200
Subject: [PATCH] locking/rwsem: Fix down_write_killable()

The new signal_pending exit path in __rwsem_down_write_failed_common()
was fingered as breaking his kernel by Tetsuo Handa.

Upon inspection it was found that there are two things wrong with it;

 - it forgets to remove WAITING_BIAS if it leaves the list empty, or
 - it forgets to wake further waiters that were blocked on the now
   removed waiter.

Especially the first issue causes new lock attempts to block and stall
indefinitely, as the code assumes that pending waiters mean there is
an owner that will wake when it releases the lock.

Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: Waiman Long <Waiman.Long@hpe.com>
Link: http://lkml.kernel.org/r/20160512115745.GP3192@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/rwsem-xadd.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index df4dcb8..09e30c6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -487,23 +487,32 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
 
 		/* Block until there are no active lockers. */
 		do {
-			if (signal_pending_state(state, current)) {
-				raw_spin_lock_irq(&sem->wait_lock);
-				ret = ERR_PTR(-EINTR);
-				goto out;
-			}
+			if (signal_pending_state(state, current))
+				goto out_nolock;
+
 			schedule();
 			set_current_state(state);
 		} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
 
 		raw_spin_lock_irq(&sem->wait_lock);
 	}
-out:
 	__set_current_state(TASK_RUNNING);
 	list_del(&waiter.list);
 	raw_spin_unlock_irq(&sem->wait_lock);
 
 	return ret;
+
+out_nolock:
+	__set_current_state(TASK_RUNNING);
+	raw_spin_lock_irq(&sem->wait_lock);
+	list_del(&waiter.list);
+	if (list_empty(&sem->wait_list))
+		rwsem_atomic_update(-RWSEM_WAITING_BIAS, sem);
+	else
+		__rwsem_do_wake(sem, RWSEM_WAKE_ANY);
+	raw_spin_unlock_irq(&sem->wait_lock);
+
+	return ERR_PTR(-EINTR);
 }
 
 __visible struct rw_semaphore * __sched
-- 
2.7.4