quota: Fix deadlock with suspend and quotas

This script causes a kernel deadlock:
set -e
DEVICE=/dev/vg1/linear
lvchange -ay $DEVICE
mkfs.ext3 $DEVICE
mount -t ext3 -o usrquota,grpquota $DEVICE /mnt/test
quotacheck -gu /mnt/test
umount /mnt/test
mount -t ext3 -o usrquota,grpquota $DEVICE /mnt/test
quotaon /mnt/test
dmsetup suspend $DEVICE
setquota -u root 1 2 3 4 /mnt/test &
sleep 1
dmsetup resume $DEVICE

setquota acquired semaphore s_umount for read and then tried to perform a
transaction (and waits because the device is suspended).  dmsetup resume tries
to acquire s_umount for write before resuming the device (and waits for
setquota).

Fix the deadlock by grabbing a thawed superblock for quota commands which need
it.

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 21 insertions, 3 deletions

diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index 7898cd688a00..fc2c4388d126 100644
--- a/fs/quota/quota.c
+++ b/fs/quota/quota.c
@@ -292,11 +292,26 @@ static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id,
         }
 }
 
+/* Return 1 if 'cmd' will block on frozen filesystem */
+static int quotactl_cmd_write(int cmd)
+{
+        switch (cmd) {
+        case Q_GETFMT:
+        case Q_GETINFO:
+        case Q_SYNC:
+        case Q_XGETQSTAT:
+        case Q_XGETQUOTA:
+        case Q_XQUOTASYNC:
+                return 0;
+        }
+        return 1;
+}
+
 /*
  * look up a superblock on which quota ops will be performed
  * - use the name of a block device to find the superblock thereon
  */
-static struct super_block *quotactl_block(const char __user *special)
+static struct super_block *quotactl_block(const char __user *special, int cmd)
 {
 #ifdef CONFIG_BLOCK
         struct block_device *bdev;
@@ -309,7 +324,10 @@ static struct super_block *quotactl_block(const char __user *special)
         putname(tmp);
         if (IS_ERR(bdev))
                 return ERR_CAST(bdev);
-        sb = get_super(bdev);
+        if (quotactl_cmd_write(cmd))
+                sb = get_super_thawed(bdev);
+        else
+                sb = get_super(bdev);
         bdput(bdev);
         if (!sb)
                 return ERR_PTR(-ENODEV);
@@ -361,7 +379,7 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
                         pathp = &path;
         }
 
-        sb = quotactl_block(special);
+        sb = quotactl_block(special, cmds);
         if (IS_ERR(sb)) {
                 ret = PTR_ERR(sb);
                 goto out;