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fs/namespace: Boost the mount_lock.lock owner instead of spinning on PREEMPT_RT.

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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.de
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
This commit is contained in:
Sebastian Andrzej Siewior 2021-11-25 13:07:11 +01:00 committed by Christian Brauner
parent 1360572566
commit 0f8821da48
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GPG Key ID: 91C61BC06578DCA2
1 changed files with 18 additions and 2 deletions
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20
fs/namespace.c
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@ -343,8 +343,24 @@ int __mnt_want_write(struct vfsmount *m)
* incremented count after it has set MNT_WRITE_HOLD.
*/
smp_mb();
while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
cpu_relax();
might_lock(&mount_lock.lock);
while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD) {
if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
cpu_relax();
} else {
/*
* This prevents priority inversion, if the task
* setting MNT_WRITE_HOLD got preempted on a remote
* CPU, and it prevents life lock if the task setting
* MNT_WRITE_HOLD has a lower priority and is bound to
* the same CPU as the task that is spinning here.
*/
preempt_enable();
lock_mount_hash();
unlock_mount_hash();
preempt_disable();
}
}
/*
* After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
* be set to match its requirements. So we must not load that until