block: fix module reference leak on put_disk() call for cgroups throttle

get_disk(),get_gendisk() calls have non explicit side effect: they
increase the reference on the disk owner module.

The following is the correct sequence how to get a disk reference and
to put it:

    disk = get_gendisk(...);

    /* use disk */

    owner = disk->fops->owner;
    put_disk(disk);
    module_put(owner);

fs/block_dev.c is aware of this required module_put() call, but f.e.
blkg_conf_finish(), which is located in block/blk-cgroup.c, does not put
a module reference.  To see a leakage in action cgroups throttle config
can be used.  In the following script I'm removing throttle for /dev/ram0
(actually this is NOP, because throttle was never set for this device):

    # lsmod | grep brd
    brd                     5175  0
    # i=100; while [ $i -gt 0 ]; do echo "1:0 0" > \
        /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device; i=$(($i - 1)); \
    done
    # lsmod | grep brd
    brd                     5175  100

Now brd module has 100 references.

The issue is fixed by calling module_put() just right away put_disk().

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Gi-Oh Kim <gi-oh.kim@profitbricks.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 39a169b62b415390398291080dafe63aec751e0a)
Signed-off-by: Alex Shi <alex.shi@linaro.org>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 9d359e05fad74d0a2f2a9a64ef3aeeb40bb3a7d6..8161090a19708684224402c59bfdb55dbbc07ef4 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -788,6 +788,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 {
        struct gendisk *disk;
        struct blkcg_gq *blkg;
+       struct module *owner;
        unsigned int major, minor;
        int key_len, part, ret;
        char *body;
@@ -804,7 +805,9 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
        if (!disk)
                return -ENODEV;
        if (part) {
+               owner = disk->fops->owner;
                put_disk(disk);
+               module_put(owner);
                return -ENODEV;
        }
 
@@ -820,7 +823,9 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                ret = PTR_ERR(blkg);
                rcu_read_unlock();
                spin_unlock_irq(disk->queue->queue_lock);
+               owner = disk->fops->owner;
                put_disk(disk);
+               module_put(owner);
                /*
                 * If queue was bypassing, we should retry.  Do so after a
                 * short msleep().  It isn't strictly necessary but queue
@@ -851,9 +856,13 @@ EXPORT_SYMBOL_GPL(blkg_conf_prep);
 void blkg_conf_finish(struct blkg_conf_ctx *ctx)
        __releases(ctx->disk->queue->queue_lock) __releases(rcu)
 {
+       struct module *owner;
+
        spin_unlock_irq(ctx->disk->queue->queue_lock);
        rcu_read_unlock();
+       owner = ctx->disk->fops->owner;
        put_disk(ctx->disk);
+       module_put(owner);
 }
 EXPORT_SYMBOL_GPL(blkg_conf_finish);