locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER_UNKNOWN

The filesystem freezing code needs to transfer ownership of a rwsem
embedded in a percpu-rwsem from the task that does the freezing to
another one that does the thawing by calling percpu_rwsem_release()
after freezing and percpu_rwsem_acquire() before thawing.

However, the new rwsem debug code runs afoul with this scheme by warning
that the task that releases the rwsem isn't the one that acquires it,
as reported by Amir Goldstein:

DEBUG_LOCKS_WARN_ON(sem->owner != get_current())
WARNING: CPU: 1 PID: 1401 at /home/amir/build/src/linux/kernel/locking/rwsem.c:133 up_write+0x59/0x79

Call Trace:
percpu_up_write+0x1f/0x28
thaw_super_locked+0xdf/0x120
do_vfs_ioctl+0x270/0x5f1
ksys_ioctl+0x52/0x71
__x64_sys_ioctl+0x16/0x19
do_syscall_64+0x5d/0x167
entry_SYSCALL_64_after_hwframe+0x49/0xbe

To work properly with the rwsem debug code, we need to annotate that the
rwsem ownership is unknown during the tranfer period until a brave soul
comes forward to acquire the ownership. During that period, optimistic
spinning will be disabled.

Reported-by: Amir Goldstein <[e-mail]>
Tested-by: Amir Goldstein <[e-mail]>
Signed-off-by: Waiman Long <[e-mail]>
Acked-by: Peter Zijlstra <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Davidlohr Bueso <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Paul E. McKenney <[e-mail]>
Cc: Theodore Y. Ts'o <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

include/linux/percpu-rwsem.h

···
 	lock_release(&sem->rw_sem.dep_map, 1, ip);
 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 	if (!read)
-		sem->rw_sem.owner = NULL;
+		sem->rw_sem.owner = RWSEM_OWNER_UNKNOWN;
 #endif
 }
 
···
 					bool read, unsigned long ip)
 {
 	lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+	if (!read)
+		sem->rw_sem.owner = current;
+#endif
 }
 
 #endif

include/linux/rwsem.h

···
 #endif
 };
 
+/*
+ * Setting bit 0 of the owner field with other non-zero bits will indicate
+ * that the rwsem is writer-owned with an unknown owner.
+ */
+#define RWSEM_OWNER_UNKNOWN	((struct task_struct *)-1L)
+
 extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
 extern struct rw_semaphore *rwsem_down_read_failed_killable(struct rw_semaphore *sem);
 extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);

kernel/locking/rwsem-xadd.c

···
 	struct task_struct *owner;
 	bool ret = true;
 
+	BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
+
 	if (need_resched())
 		return false;