lockdep: fix oops in processing workqueue (4d82a1de) · Commits · e / devices / android_kernel_xiaomi_markw

Under memory load, on x86_64, with lockdep enabled, the workqueue's process_one_work() has been seen to oops in __lock_acquire(), barfing on a 0xffffffff00000000 pointer in the lockdep_map's class_cache[]. Because it's permissible to free a work_struct from its callout function, the map used is an onstack copy of the map given in the work_struct: and that copy is made without any locking. Surprisingly, gcc (4.5.1 in Hugh's case) uses "rep movsl" rather than "rep movsq" for that structure copy: which might race with a workqueue user's wait_on_work() doing lock_map_acquire() on the source of the copy, putting a pointer into the class_cache[], but only in time for the top half of that pointer to be copied to the destination map. Boom when process_one_work() subsequently does lock_map_acquire() on its onstack copy of the lockdep_map. Fix this, and a similar instance in call_timer_fn(), with a lockdep_copy_map() function which additionally NULLs the class_cache[]. Note: this oops was actually seen on 3.4-next, where flush_work() newly does the racing lock_map_acquire(); but Tejun points out that 3.4 and earlier are already vulnerable to the same through wait_on_work(). * Patch orginally from Peter. Hugh modified it a bit and wrote the description. Signed-off-by:

Peter Zijlstra <peterz@infradead.org> Reported-by:

Hugh Dickins <hughd@google.com> LKML-Reference: <alpine.LSU.2.00.1205070951170.1544@eggly.anvils> Signed-off-by:

Tejun Heo <tj@kernel.org>

include/linux/lockdep.h

+18 −0

Original line number	Diff line number	Diff line
		@@ -157,6 +157,24 @@ struct lockdep_map {
		#endif
		};

		static inline void lockdep_copy_map(struct lockdep_map *to,
		struct lockdep_map *from)
		{
		int i;

		to = from;
		/*
		* Since the class cache can be modified concurrently we could observe
		* half pointers (64bit arch using 32bit copy insns). Therefore clear
		* the caches and take the performance hit.
		*
		* XXX it doesn't work well with lockdep_set_class_and_subclass(), since
		* that relies on cache abuse.
		*/
		for (i = 0; i < NR_LOCKDEP_CACHING_CLASSES; i++)
		to->class_cache[i] = NULL;
		}

		/*
		* Every lock has a list of other locks that were taken after it.
		* We only grow the list, never remove from it:

kernel/timer.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -1102,7 +1102,9 @@ static void call_timer_fn(struct timer_list timer, void (fn)(unsigned long),
		* warnings as well as problems when looking into
		* timer->lockdep_map, make a copy and use that here.
		*/
		struct lockdep_map lockdep_map = timer->lockdep_map;
		struct lockdep_map lockdep_map;

		lockdep_copy_map(&lockdep_map, &timer->lockdep_map);
		#endif
		/*
		* Couple the lock chain with the lock chain at

kernel/workqueue.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -1818,7 +1818,9 @@ __acquires(&gcwq->lock)
		* lock freed" warnings as well as problems when looking into
		* work->lockdep_map, make a copy and use that here.
		*/
		struct lockdep_map lockdep_map = work->lockdep_map;
		struct lockdep_map lockdep_map;

		lockdep_copy_map(&lockdep_map, &work->lockdep_map);
		#endif
		/*
		* A single work shouldn't be executed concurrently by