linux-kernel - Re: [PATCH v2 2/2] ovl: enable RCU'd ->get

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Date:   Wed, 18 Aug 2021 11:34:31 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-unionfs@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        garyhuang <zjh.20052005@....com>
Subject: Re: [PATCH v2 2/2] ovl: enable RCU'd ->get_acl()

On Wed, Aug 18, 2021 at 6:34 AM Miklos Szeredi <mszeredi@...hat.com> wrote:
>
>  struct posix_acl *get_cached_acl_rcu(struct inode *inode, int type)
>  {
> -       return rcu_dereference(*acl_by_type(inode, type));
> +       struct posix_acl *acl = rcu_dereference(*acl_by_type(inode, type));
> +
> +       if (acl == ACL_DONT_CACHE)
> +               acl = inode->i_op->get_acl(inode, type, LOOKUP_RCU);
> +
> +       return acl;
>  }

What? No.

You just made get_cached_acl_rcu() return ERR_PTR(-EINVAL) for most filesystems.

So now you've changed the behavior of get_cached_acl_rcu() ENTIRELY.

It used to return either
 (a) the ACL
 (b) NULL
 (c) ACL_DONT_CACHE/ACL_NOT_CACHED

but now you've changed that (c) case to "ACL_NOT_CACHED or random error value".

You can't just mix these kinds of entirely different return values like that.

So no, this is not at all acceptable.

I would suggest:

 (a) make the first patch actually test explicitly for LOOKUP_RCU, so
that it's clear to the filesystems what is going on.

     So instead of that pattern of

        if (flags)
                return ERR_PTR(-EINVAL);

     I'd suggest using

        if (flags & LOOKUP_RCU)
                return ERR_PTR(-ECHILD);

   so that it actually matches what lookup does for the "I can't do
this under RCU", and so that any reader of the code understands what
"flags" is all about.

And then

 (b) make the get_cached_acl_rcu() case handle errors _properly_
instead of mixing the special ACL cache markers with error returns.

     So instead of

        if (acl == ACL_DONT_CACHE)
                acl = inode->i_op->get_acl(inode, type, LOOKUP_RCU);

     maybe something more along the lines of

        if (acl == ACL_DONT_CACHE) {
                struct posix_acl *lookup_acl;
                lookup_acl = inode->i_op->get_acl(inode, type, LOOKUP_RCU);
                if (!IS_ERR(lookup_acl))
                        acl = lookup_acl;
        }

     or whatever.

I disagree with Al that a "bool" would be better. I think LOOKUP_RCU
is good documentation, and consistent with lookup, but it really needs
to be *consistent*.  Thus that

        if (flags & LOOKUP_RCU)
                return ERR_PTR(-ECHILD);

pattern, not some "test underscibed flags, return -EINVAL" pattern
that looks entirely nonsensical.

               Linus