quota: fix livelock between quotactl and freeze_super

In the Linux kernel, the following vulnerability has been resolved:

quota: fix livelock between quotactl and freeze_super

When a filesystem is frozen, quotactl_block() enters a retry loop

waiting for the filesystem to thaw. It acquires s_umount, checks the

freeze state, drops s_umount and uses sb_start_write() - sb_end_write()

pair to wait for the unfreeze.

However, this retry loop can trigger a livelock issue, specifically on

kernels with preemption disabled.

The mechanism is as follows:

1. freeze_super() sets SB_FREEZE_WRITE and calls sb_wait_write().

2. sb_wait_write() calls percpu_down_write(), which initiates

synchronize_rcu().

3. Simultaneously, quotactl_block() spins in its retry loop, immediately

executing the sb_start_write() - sb_end_write() pair.

4. Because the kernel is non-preemptible and the loop contains no

scheduling points, quotactl_block() never yields the CPU. This

prevents that CPU from reaching an RCU quiescent state.

5. synchronize_rcu() in the freezer thread waits indefinitely for the

quotactl_block() CPU to report a quiescent state.

6. quotactl_block() spins indefinitely waiting for the freezer to

advance, which it cannot do as it is blocked on the RCU sync.

This results in a hang of the freezer process and 100% CPU usage by the

quota process.

While this can occur intermittently on multi-core systems, it is

reliably reproducing on a node with the following script, running both

the freezer and the quota toggle on the same CPU:

# mkfs.ext4 -O quota /dev/sda 2g && mkdir a_mount

# mount /dev/sda -o quota,usrquota,grpquota a_mount

# taskset -c 3 bash -c "while true; do xfs_freeze -f a_mount; \

xfs_freeze -u a_mount; done" &

# taskset -c 3 bash -c "while true; do quotaon a_mount; \

quotaoff a_mount; done" &

Adding cond_resched() to the retry loop fixes the issue. It acts as an

RCU quiescent state, allowing synchronize_rcu() in percpu_down_write()

to complete.