On Fri, 2022-02-18 at 10:08 -0600, Eric W. Biederman wrote:
>
> Maybe I am reading the lifetimes wrong but is there
> any chance the code can just do something like the diff below?
>
> AKA have a special version of kern_umount that does the call_rcu?
>
> Looking at rcu_reclaim_tiny I think this use of mnt_rcu is valid.
> AKA reusing the rcu_head in the rcu callback.
>
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 40b994a29e90..7d7aaef1592e 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -4395,6 +4395,22 @@ void kern_unmount(struct vfsmount *mnt)
> }
> EXPORT_SYMBOL(kern_unmount);
>
> +static void rcu_mntput(struct rcu_head *head)
> +{
> + struct mount *mnt = container_of(head, struct mount,
> mnt_rcu);
> + mntput(&mnt->mnt);
> +}
> +
> +void kern_rcu_unmount(struct vfsmount *mnt)
> +{
> + /* release long term mount so mount point can be released */
> + if (!IS_ERR_OR_NULL(mnt)) {
> + struct mount *m = real_mount(mnt);
> + m->mnt_ns = NULL;
> + call_rcu(&m->mnt_rcu, rcu_mntput);
> + }
> +}
OK, two comments here:
1) As Paul pointed out, we need to use call_rcu_work here,
because rcu_mntput needs to run from a work item (or well,
any process context) because of the rwlock.
2) No user of kern_unmount can use the vfsmount structure
after kern_unmount returns. That means they could all use
the RCU version, and no special version is needed.
Let me go test a patch that does that...
--
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