Re: 3.14.0+/x86: lockdep and mutexes not getting along

[Jason Low]

On Wed, 2014-04-09 at 15:19 +0300, Kirill A. Shutemov wrote:
> On Sun, Apr 06, 2014 at 01:12:14AM -0400, Michael L. Semon wrote:
> > Hi!  Starting early in this merge window for 3.15, lockdep has been
> > giving me trouble.  Normally, a splat will happen, lockdep will shut
> > itself off, and my i686 Pentium 4 PC will continue.  Now, after the
> > splat, it will allow one key of input at either a VGA console or over
> > serial.  After that, only the magic SysRq keys and KDB still work.
> > File activity stops, and many processes are stuck in the D state.
> > 
> > Bisect brought me here:
> > 
> > root@plbearer:/usr/src/kernel-git/linux# git bisect good
> > 6f008e72cd111a119b5d8de8c5438d892aae99eb is the first bad commit
> > commit 6f008e72cd111a119b5d8de8c5438d892aae99eb
> > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Date:   Wed Mar 12 13:24:42 2014 +0100
> > 
> >     locking/mutex: Fix debug checks
> > 
> >     OK, so commit:
> > 
> >       1d8fe7dc8078 ("locking/mutexes: Unlock the mutex without the wait_lock")
> > 
> >     generates this boot warning when CONFIG_DEBUG_MUTEXES=y:
> > 
> >       WARNING: CPU: 0 PID: 139 at /usr/src/linux-2.6/kernel/locking/mutex-debug.c:82 debug_mutex_unlock+0x155/0x180() DEBUG_LOCKS_WARN_ON(lock->owner != current)
> > 
> >     And that makes sense, because as soon as we release the lock a
> >     new owner can come in...
> > 
> >     One would think that !__mutex_slowpath_needs_to_unlock()
> >     implementations suffer the same, but for DEBUG we fall back to
> >     mutex-null.h which has an unconditional 1 for that.
> > 
> >     The mutex debug code requires the mutex to be unlocked after
> >     doing the debug checks, otherwise it can find inconsistent
> >     state.
> > 
> >     Reported-by: Ingo Molnar <mingo@xxxxxxxxxx>
> >     Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >     Cc: jason.low2@xxxxxx

Hello,

As a starting point, would either of you like to test the following
patch to see if it fixes the issue? This patch essentially generates the
same code as in older kernels in the debug case. This applies on top of
kernels with both commits 6f008e72cd11 and 1d8fe7dc8078.

Thanks.

-----
diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index e1191c9..faf6f5b 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -83,12 +83,6 @@ void debug_mutex_unlock(struct mutex *lock)
 
 	DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
 	mutex_clear_owner(lock);
-
-	/*
-	 * __mutex_slowpath_needs_to_unlock() is explicitly 0 for debug
-	 * mutexes so that we can do it here after we've verified state.
-	 */
-	atomic_set(&lock->count, 1);
 }
 
 void debug_mutex_init(struct mutex *lock, const char *name,
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index bc73d33..f1f672e 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -34,13 +34,6 @@
 #ifdef CONFIG_DEBUG_MUTEXES
 # include "mutex-debug.h"
 # include <asm-generic/mutex-null.h>
-/*
- * Must be 0 for the debug case so we do not do the unlock outside of the
- * wait_lock region. debug_mutex_unlock() will do the actual unlock in this
- * case.
- */
-# undef __mutex_slowpath_needs_to_unlock
-# define  __mutex_slowpath_needs_to_unlock()	0
 #else
 # include "mutex.h"
 # include <asm/mutex.h>
@@ -688,6 +681,17 @@ __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
 	unsigned long flags;
 
 	/*
+	 * In the debug cases, obtain the wait_lock first
+	 * before calling the following debugging functions.
+	 */
+#if defined(CONFIG_DEBUG_MUTEXES) || defined(CONFIG_DEBUG_LOCK_ALLOC)
+	spin_lock_mutex(&lock->wait_lock, flags);
+#endif
+
+	mutex_release(&lock->dep_map, nested, _RET_IP_);
+	debug_mutex_unlock(lock);
+
+	/*
 	 * some architectures leave the lock unlocked in the fastpath failure
 	 * case, others need to leave it locked. In the later case we have to
 	 * unlock it here
@@ -695,9 +699,9 @@ __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
 	if (__mutex_slowpath_needs_to_unlock())
 		atomic_set(&lock->count, 1);
 
+#if !defined(CONFIG_DEBUG_MUTEXES) && !defined(CONFIG_DEBUG_LOCK_ALLOC)
 	spin_lock_mutex(&lock->wait_lock, flags);
-	mutex_release(&lock->dep_map, nested, _RET_IP_);
-	debug_mutex_unlock(lock);
+#endif
 
 	if (!list_empty(&lock->wait_list)) {
 		/* get the first entry from the wait-list: */


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