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. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx> Link: https://lore.kernel.org/r/20211025152218.opvcqfku2lhqvp4o@xxxxxxxxxxxxx --- fs/namespace.c | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 659a8f39c61af..3ab45b47b2860 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 -- 2.34.0
