On Mon, 27 Jul 2015 11:08:32 +0800 Kinglong Mee <kinglongmee@xxxxxxxxx>
wrote:
> New helper legitimize_mntget for getting a mnt without setting
> MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED, otherwise return NULL.
>
> v8, same as v6
>
> Signed-off-by: Kinglong Mee <kinglongmee@xxxxxxxxx>
> ---
> fs/namespace.c | 19 +++++++++++++++++++
> include/linux/mount.h | 1 +
> 2 files changed, 20 insertions(+)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 2b8aa15..842cf57 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1153,6 +1153,25 @@ struct vfsmount *mntget(struct vfsmount *mnt)
> }
> EXPORT_SYMBOL(mntget);
>
> +struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt)
> +{
> + struct mount *mnt;
> +
> + if (vfsmnt == NULL)
> + return NULL;
> +
> + read_seqlock_excl(&mount_lock);
> + mnt = real_mount(vfsmnt);
> + if (vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED))
> + vfsmnt = NULL;
> + else
> + mnt_add_count(mnt, 1);
> + read_sequnlock_excl(&mount_lock);
> +
> + return vfsmnt;
> +}
> +EXPORT_SYMBOL(legitimize_mntget);
> +
> struct vfsmount *mnt_clone_internal(struct path *path)
> {
> struct mount *p;
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index f822c3c..8ae9dc0 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -79,6 +79,7 @@ extern void mnt_drop_write(struct vfsmount *mnt);
> extern void mnt_drop_write_file(struct file *file);
> extern void mntput(struct vfsmount *mnt);
> extern struct vfsmount *mntget(struct vfsmount *mnt);
> +extern struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt);
> extern struct vfsmount *mnt_clone_internal(struct path *path);
> extern int __mnt_is_readonly(struct vfsmount *mnt);
>
It is unfortunate that we seem to have to take the mount_lock global
lock on every nfs request. I wonder if we can avoid that....
What if we did:
seq = 0;
retry:
read_seqbegin_or_lock(&mount_lock, &seq);
if (vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED))
vfsmnt = NULL;
else if (need_seqretry(&mount_lock, seq);
goto retry;
else {
mnt_add_count(&mnt, 1);
if (need_seqretry(&mount_lock, seq) ||
vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT |
MNT_DOOMED)) {
mnt_add_count(&mnt, -1);
goto retry;
}
}
done_seqretry(&mount_lock, seq);
Is there any risk from having a temporarily elevated mnt_count there?
I can't see one, but there is clearly some complexity in managing that
count.
Thanks,
NeilBrown
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