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WRITE_HOLD machinery: no need for to bump mount_lock seqcount

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... neither for insertion into the list of instances, nor for
mnt_{un,}hold_writers(), nor for mnt_get_write_access() deciding
to be nice to RT during a busy-wait loop - all of that only needs
the spinlock side of mount_lock.

IOW, it's mount_locked_reader, not mount_writer.

Clarify the comment re locking rules for mnt_unhold_writers() - it's
not just that mount_lock needs to be held when calling that, it must
have been held all along since the matching mnt_hold_writers().

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro
2025-08-27 14:04:31 -04:00
parent 3371fa2f27
commit 1e414adf03
1 changed files with 8 additions and 8 deletions
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16
fs/namespace.c
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@@ -526,8 +526,8 @@ int mnt_get_write_access(struct vfsmount *m)
* the same CPU as the task that is spinning here.
*/
preempt_enable();
lock_mount_hash();
unlock_mount_hash();
read_seqlock_excl(&mount_lock);
read_sequnlock_excl(&mount_lock);
preempt_disable();
}
}
@@ -671,7 +671,7 @@ EXPORT_SYMBOL(mnt_drop_write_file);
* a call to mnt_unhold_writers() in order to stop preventing write access to
* @mnt.
*
* Context: This function expects lock_mount_hash() to be held serializing
* Context: This function expects to be in mount_locked_reader scope serializing
* setting WRITE_HOLD.
* Return: On success 0 is returned.
* On error, -EBUSY is returned.
@@ -716,7 +716,8 @@ static inline int mnt_hold_writers(struct mount *mnt)
*
* This function can only be called after a call to mnt_hold_writers().
*
* Context: This function expects lock_mount_hash() to be held.
* Context: This function expects to be in the same mount_locked_reader scope
* as the matching mnt_hold_writers().
*/
static inline void mnt_unhold_writers(struct mount *mnt)
{
@@ -770,7 +771,8 @@ int sb_prepare_remount_readonly(struct super_block *sb)
if (atomic_long_read(&sb->s_remove_count))
return -EBUSY;
lock_mount_hash();
guard(mount_locked_reader)();
for (struct mount *m = sb->s_mounts; m; m = m->mnt_next_for_sb) {
if (!(m->mnt.mnt_flags & MNT_READONLY)) {
err = mnt_hold_writers(m);
@@ -787,7 +789,6 @@ int sb_prepare_remount_readonly(struct super_block *sb)
if (test_write_hold(m))
clear_write_hold(m);
}
unlock_mount_hash();
return err;
}
@@ -1226,9 +1227,8 @@ static void setup_mnt(struct mount *m, struct dentry *root)
m->mnt_mountpoint = m->mnt.mnt_root;
m->mnt_parent = m;
lock_mount_hash();
guard(mount_locked_reader)();
mnt_add_instance(m, s);
unlock_mount_hash();
}
/**