4.14-stable review patch. If anyone has any objections, please let me know.
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From: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp
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=b297474817af98d5796bc544e1bb806fc3da0e5...
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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 @@ -259,4 +259,21 @@ int wait_on_atomic_t(atomic_t *val, int 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 @@ -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)