From 8236b0ae31c837d2b3a2565c5f8d77f637e824cc Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Wed, 2 May 2018 07:07:55 +0900
Subject: [PATCH] bdi: wake up concurrent wb_shutdown() callers.
Git-commit: 8236b0ae31c837d2b3a2565c5f8d77f637e824cc
Patch-mainline: v4.17-rc4
References: bsc#1052766
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@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+c0cf869505e03bdf1a24@syzkaller.appspotmail.com>
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Jan Kara <jack@suse.cz>
---
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
@@ -327,4 +327,21 @@ do { \
__ret; \
})
+/**
+ * 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
@@ -381,7 +381,7 @@ 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)