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| author | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2021-11-25 13:07:11 +0100 |
|---|---|---|
| committer | Christian Brauner <christian.brauner@ubuntu.com> | 2021-11-26 12:09:09 +0100 |
| commit | 0f8821da48458982cf379eb4432f23958f2e3a6c (patch) | |
| tree | 95089eb557bdf7a7e67a1396819052d074ae1ec5 /fs | |
| parent | 136057256686de39cc3a07c2e39ef6bc43003ff6 (diff) | |
| download | lwn-0f8821da48458982cf379eb4432f23958f2e3a6c.tar.gz lwn-0f8821da48458982cf379eb4432f23958f2e3a6c.zip | |
fs/namespace: Boost the mount_lock.lock owner instead of spinning on PREEMPT_RT.
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>
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/namespace.c | 20 |
1 files changed, 18 insertions, 2 deletions
diff --git a/fs/namespace.c b/fs/namespace.c index 659a8f39c61a..3ab45b47b286 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -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 |