4.16-stable review patch. If anyone has any objections, please let me know.
------------------
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
commit 8236b0ae31c837d2b3a2565c5f8d77f637e824cc upstream.
syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
wb_shutdown() [1]. This seems to be because commit 5318ce7d46866e1d ("bdi:
Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).
Introduce a helper function clear_and_wake_up_bit() and use it, in order
to avoid similar errors in future.
[1] https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+c0cf869505e03bdf1a24@xxxxxxxxxxxxxxxxxxxxxxxxx>
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
include/linux/wait_bit.h | 17 +++++++++++++++++
mm/backing-dev.c | 2 +-
2 files changed, 18 insertions(+), 1 deletion(-)
--- a/include/linux/wait_bit.h
+++ b/include/linux/wait_bit.h
@@ -262,4 +262,21 @@ int wait_on_atomic_t(atomic_t *val, wait
return out_of_line_wait_on_atomic_t(val, action, mode);
}
+/**
+ * clear_and_wake_up_bit - clear a bit and wake up anyone waiting on that bit
+ *
+ * @bit: the bit of the word being waited on
+ * @word: the word being waited on, a kernel virtual address
+ *
+ * You can use this helper if bitflags are manipulated atomically rather than
+ * non-atomically under a lock.
+ */
+static inline void clear_and_wake_up_bit(int bit, void *word)
+{
+ clear_bit_unlock(bit, word);
+ /* See wake_up_bit() for which memory barrier you need to use. */
+ smp_mb__after_atomic();
+ wake_up_bit(word, bit);
+}
+
#endif /* _LINUX_WAIT_BIT_H */
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -394,7 +394,7 @@ static void wb_shutdown(struct bdi_write
* the barrier provided by test_and_clear_bit() above.
*/
smp_wmb();
- clear_bit(WB_shutting_down, &wb->state);
+ clear_and_wake_up_bit(WB_shutting_down, &wb->state);
}
static void wb_exit(struct bdi_writeback *wb)
