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| author | NeilBrown <neilb@suse.com> | 2016-12-12 08:21:51 -0700 |
|---|---|---|
| committer | Jens Axboe <axboe@fb.com> | 2016-12-13 21:07:08 -0700 |
| commit | bcc7f5b4bee8e327689a4d994022765855c807ff (patch) | |
| tree | 2e233899a43c69172710876cf626a57139616392 /fs/block_dev.c | |
| parent | f4000cd99750065d5177555c0a805c97174d1b9f (diff) | |
| download | lwn-bcc7f5b4bee8e327689a4d994022765855c807ff.tar.gz lwn-bcc7f5b4bee8e327689a4d994022765855c807ff.zip | |
block_dev: don't test bdev->bd_contains when it is not stable
bdev->bd_contains is not stable before calling __blkdev_get().
When __blkdev_get() is called on a parition with ->bd_openers == 0
it sets
bdev->bd_contains = bdev;
which is not correct for a partition.
After a call to __blkdev_get() succeeds, ->bd_openers will be > 0
and then ->bd_contains is stable.
When FMODE_EXCL is used, blkdev_get() calls
bd_start_claiming() -> bd_prepare_to_claim() -> bd_may_claim()
This call happens before __blkdev_get() is called, so ->bd_contains
is not stable. So bd_may_claim() cannot safely use ->bd_contains.
It currently tries to use it, and this can lead to a BUG_ON().
This happens when a whole device is already open with a bd_holder (in
use by dm in my particular example) and two threads race to open a
partition of that device for the first time, one opening with O_EXCL and
one without.
The thread that doesn't use O_EXCL gets through blkdev_get() to
__blkdev_get(), gains the ->bd_mutex, and sets bdev->bd_contains = bdev;
Immediately thereafter the other thread, using FMODE_EXCL, calls
bd_start_claiming() from blkdev_get(). This should fail because the
whole device has a holder, but because bdev->bd_contains == bdev
bd_may_claim() incorrectly reports success.
This thread continues and blocks on bd_mutex.
The first thread then sets bdev->bd_contains correctly and drops the mutex.
The thread using FMODE_EXCL then continues and when it calls bd_may_claim()
again in:
BUG_ON(!bd_may_claim(bdev, whole, holder));
The BUG_ON fires.
Fix this by removing the dependency on ->bd_contains in
bd_may_claim(). As bd_may_claim() has direct access to the whole
device, it can simply test if the target bdev is the whole device.
Fixes: 6b4517a7913a ("block: implement bd_claiming and claiming block")
Cc: stable@vger.kernel.org (v2.6.35+)
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Diffstat (limited to 'fs/block_dev.c')
| -rw-r--r-- | fs/block_dev.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/fs/block_dev.c b/fs/block_dev.c index 95acbd2ebc5d..d4f23353ee4d 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -1089,7 +1089,7 @@ static bool bd_may_claim(struct block_device *bdev, struct block_device *whole, return true; /* already a holder */ else if (bdev->bd_holder != NULL) return false; /* held by someone else */ - else if (bdev->bd_contains == bdev) + else if (whole == bdev) return true; /* is a whole device which isn't held */ else if (whole->bd_holder == bd_may_claim) |