On Mon, Feb 26, 2018 at 6:53 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
>
> So the purpose for having a patch in the first place is that
> 2a3a2a3f3524 ("ovl: don't cache acl on overlay layer")
> which addded ACL_DONT_CACHED did not result in any comment updates
> to get_acl.
I'm not opposed to just updating the comments.
I just think your updates were somewhat misleading.
> Which mean that if you read the comments in get_acl() that you
> don't even think of ACL_DONT_CACHED.
Right. By all means add a comment about ACL_DONT_CACHE disabling the
cache entirely.
But don't _remove_ the other valid way to flush the cache, and don't
make that comment above cmpxchg() be even more confusing than the code
is.
> Does this look better as a comment updating patch?
>
> diff --git a/fs/posix_acl.c b/fs/posix_acl.c
> index 2fd0fde16fe1..5453094b8828 100644
> --- a/fs/posix_acl.c
> +++ b/fs/posix_acl.c
> @@ -98,6 +98,11 @@ struct posix_acl *get_acl(struct inode *inode, int type)
> struct posix_acl **p;
> struct posix_acl *acl;
>
> + /*
> + * To avoid caching the result of ->get_acl
> + * set inode->i_acl = inode->i_default_acl = ACL_DONT_CACHE;
> + */
> +
> /*
> * The sentinel is used to detect when another operation like
> * set_cached_acl() or forget_cached_acl() races with get_acl().
> @@ -126,9 +131,7 @@ struct posix_acl *get_acl(struct inode *inode, int type)
> /* fall through */ ;
>
> /*
> - * Normally, the ACL returned by ->get_acl will be cached.
> - * A filesystem can prevent that by calling
> - * forget_cached_acl(inode, type) in ->get_acl.
> + * The ACL returned by ->get_acl will be cached.
Why do you hate forget_cached_acl()?
It's perfectly valid too. Don't remove that comment. Maybe reword it
to talk not about "preventing", but about "invalidating the cache".
But the old comment that you remove isn't _wrong_, it's just that the
"preventing" from returning the cached state with forget_cached_acl()
is just a one-time thing.
So forget_cached_acl() exists, and it works, and it does exactly what
its name says. It is a perfectly valid way to prevent the current
entry from being used in the future.
See? I object to you removing that, and trying to make it be like
ACL_DONT_CACHE is the *onyl* way to not cache something.
Because honestly, that's what your comment updates do. They take the
comments about _one_ case, and switch it over to be about the _othger_
case.
But dammit, there are _two_ ways to not cache things.
"Fixing" the comment to talk about one and removing the other isn't a
fix. It's just a stupid change that now has the problem the other way
around!
So fix the comment to really just talk about both things.
First: talk about how to avoid caching entirely (ACL_DONT_CACHE).
Then, talk about how to invalidate the cache once it has been
instantiated (forget_cached_acl()).
Don't do this idiotic "remove the valid comment just because you
happened to care about the _other_ case"
Linus
