The MNT_WRITE_HOLD flag is used to hold back any new writers while the
mount point is about to be made read-only. __mnt_want_write() then loops
with disabled preemption until this flag disappears. Callers of
mnt_hold_writers() (which sets the flag) hold the spinlock_t of
mount_lock (seqlock_t) which disables preemption on !PREEMPT_RT and
ensures the task is not scheduled away so that the spinning side spins
for a long time.

On PREEMPT_RT the spinlock_t does not disable preemption and so it is
possible that the task setting MNT_WRITE_HOLD is preempted by task with
higher priority which then spins infinitely waiting for MNT_WRITE_HOLD
to get removed.

Acquire mount_lock::lock which is held by setter of MNT_WRITE_HOLD. This
will PI-boost the owner and wait until the lock is dropped and which
means that MNT_WRITE_HOLD is cleared again.

Link: https://lore.kernel.org/r/20211025152218.opvcqfku2lhqvp4o@linutronix.de
Link: https://lore.kernel.org/r/20211125120711.dgbsienyrsxfzpoi@linutronix.deAcked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>