[PATCH] Avoid lost wakeups in lock_page_killable()

[Chris Mason <chris.mason@oracle.com>]

lock_page and lock_page_killable both call __wait_on_bit_lock, and
both end up using prepare_to_wait_exclusive().  This means that when
someone does finally unlock the page, only one process is going to get
woken up.

But lock_page_killable can exit without taking the lock.  If nobody
else comes in and locks the page, any other waiters will wait forever.

For example, procA holding the page lock, procB and procC are waiting on
the lock.

procA: lock_page() // success
procB: lock_page_killable(), sync_page_killable(), io_schedule()
procC: lock_page_killable(), sync_page_killable(), io_schedule()

procA: unlock, wake_up_page(page, PG_locked)
procA: wake up procB

happy admin: kill procB

procB: wakes into sync_page_killable(), notices the signal and returns
-EINTR

procB: __wait_on_bit_lock sees the action() func returns < 0 and does
not take the page lock

procB: lock_page_killable() returns < 0 and exits happily.

procC: sleeping in io_schedule() forever unless someone else locks the
page.

This was seen in production on systems where the database was shutting
down.  Testing shows the patch fixes things.

Chuck Lever did all the hard work here, with a page lock debugging
patch that proved we were missing a wakeup.  

Every version of lock_page_killable() should need this.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/mm/filemap.c b/mm/filemap.c
index ceba0bd..e1184fa 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -623,9 +623,20 @@ EXPORT_SYMBOL(__lock_page);
 int __lock_page_killable(struct page *page)
 {
 	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
+	int ret;
 
-	return __wait_on_bit_lock(page_waitqueue(page), &wait,
+	ret = __wait_on_bit_lock(page_waitqueue(page), &wait,
 					sync_page_killable, TASK_KILLABLE);
+	/*
+	 * wait_on_bit_lock uses prepare_to_wait_exclusive, so if multiple
+	 * procs were waiting on this page, we were the only proc woken up.
+	 *
+	 * if ret != 0, we didn't actually get the lock.  We need to
+	 * make sure any other waiters don't sleep forever.
+	 */
+	if (ret)
+		wake_up_page(page, PG_locked);
+	return ret;
 }
 
 /**


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