block: fix mis-synchronisation in blkdev_issue_zeroout()
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@redhat.com>
Reported-by: Eric Whitney <eric.whitney@hp.com>
Tested-by: Eric Whitney <eric.whitney@hp.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
CC: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
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 @@
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 @@
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 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 @@
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))