Ping?
-- Steve
On Wed, 9 Dec 2020 17:09:28 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
>
> Running my yearly branch profiling code, it detected a 100% wrong branch
> condition in name.c for lookup_fast(). The code in question has:
>
> status = d_revalidate(dentry, nd->flags);
> if (likely(status > 0))
> return dentry;
> if (unlazy_child(nd, dentry, seq))
> return ERR_PTR(-ECHILD);
> if (unlikely(status == -ECHILD))
> /* we'd been told to redo it in non-rcu mode */
> status = d_revalidate(dentry, nd->flags);
>
> If the status of the d_revalidate() is greater than zero, then the function
> finishes. Otherwise, if it is an "unlazy_child" it returns with -ECHILD.
> After the above two checks, the status is compared to -ECHILD, as that is
> what is returned if the original d_revalidate() needed to be done in a
> non-rcu mode.
>
> Especially this path is called in a condition of:
>
> if (nd->flags & LOOKUP_RCU) {
>
> And most of the d_revalidate() functions have:
>
> if (flags & LOOKUP_RCU)
> return -ECHILD;
>
> It appears that that is the only case that this if statement is triggered
> on two of my machines, running in production.
>
> As it is dependent on what filesystem mix is configured in the running
> kernel, simply remove the unlikely() from the if statement.
>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
> Changes since v1:
>
> - Remove unlikely() instead of making it a likely()
>
> diff --git a/fs/namei.c b/fs/namei.c
> index d4a6dd772303..c7b7e83853f3 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1495,7 +1495,7 @@ static struct dentry *lookup_fast(struct nameidata *nd,
> return dentry;
> if (unlazy_child(nd, dentry, seq))
> return ERR_PTR(-ECHILD);
> - if (unlikely(status == -ECHILD))
> + if (status == -ECHILD)
> /* we'd been told to redo it in non-rcu mode */
> status = d_revalidate(dentry, nd->flags);
> } else {
