BZ29402 https://bugzilla.kernel.org/show_bug.cgi?id=29402 We can hit serious mis-synchronization in bio completion path of blkdev_issue_zeroout() leading to a panic. The problem is that when we are going to wait_for_completion() in blkdev_issue_zeroout() we check if the bb.done equals issued (number of submitted bios). If it does, we can skip the wait_for_completition() and just out of the function since there is nothing to wait for. However, there is a ordering problem because bio_batch_end_io() is calling atomic_inc(&bb->done) before complete(), hence it might seem to blkdev_issue_zeroout() that all bios has been completed and exit. At this point when bio_batch_end_io() is going to call complete(bb->wait), bb and wait does not longer exist since it was allocated on stack in blkdev_issue_zeroout() ==> panic! (thread 1) (thread 2) bio_batch_end_io() blkdev_issue_zeroout() if(bb) { ... if (bb->end_io) ... bb->end_io(bio, err); ... atomic_inc(&bb->done); ... ... while (issued != atomic_read(&bb.done)) ... (let issued == bb.done) ... (do the rest of the function) ... return ret; complete(bb->wait); ^^^^^^^^ panic We can fix this easily by simplifying bio_batch and completion counting. Also remove bio_end_io_t *end_io since it is not used. Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx> Reported-by: Eric Whitney <eric.whitney@xxxxxx> Tested-by: Eric Whitney <eric.whitney@xxxxxx> CC: Jens Axboe <axboe@xxxxxxxxx> CC: Dmitry Monakhov <dmonakhov@xxxxxxxxxx> CC: Jeff Moyer <jmoyer@xxxxxxxxxx> --- block/blk-lib.c | 19 +++++++------------ 1 files changed, 7 insertions(+), 12 deletions(-) diff --git a/block/blk-lib.c b/block/blk-lib.c index eec78be..bd3e8df 100644 --- a/block/blk-lib.c +++ b/block/blk-lib.c @@ -109,7 +109,6 @@ struct bio_batch atomic_t done; unsigned long flags; struct completion *wait; - bio_end_io_t *end_io; }; static void bio_batch_end_io(struct bio *bio, int err) @@ -122,12 +121,9 @@ static void bio_batch_end_io(struct bio *bio, int err) else clear_bit(BIO_UPTODATE, &bb->flags); } - if (bb) { - if (bb->end_io) - bb->end_io(bio, err); - atomic_inc(&bb->done); - complete(bb->wait); - } + if (bb) + if (atomic_dec_and_test(&bb->done)) + complete(bb->wait); bio_put(bio); } @@ -150,13 +146,12 @@ int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector, int ret; struct bio *bio; struct bio_batch bb; - unsigned int sz, issued = 0; + unsigned int sz; DECLARE_COMPLETION_ONSTACK(wait); - atomic_set(&bb.done, 0); + atomic_set(&bb.done, 1); bb.flags = 1 << BIO_UPTODATE; bb.wait = &wait; - bb.end_io = NULL; submit: ret = 0; @@ -185,12 +180,12 @@ submit: break; } ret = 0; - issued++; + atomic_inc(&bb.done); submit_bio(WRITE, bio); } /* Wait for bios in-flight */ - while (issued != atomic_read(&bb.done)) + if (!atomic_dec_and_test(&bb.done)) wait_for_completion(&wait); if (!test_bit(BIO_UPTODATE, &bb.flags)) -- 1.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
