linux-kernel - Re: [PATCH v7 3/7] fs/posix_acl: Document that get_acl respects ACL_DONT

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Date:   Mon, 26 Feb 2018 20:53:02 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alban Crequy <alban@...volk.io>,
        Seth Forshee <seth.forshee@...onical.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Dongsu Park <dongsu@...volk.io>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH v7 3/7] fs/posix_acl: Document that get_acl respects ACL_DONT_CACHE

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.

Which mean that if you read the comments in get_acl() that you
don't even think of ACL_DONT_CACHED.

Which means that this comment:
	/*
	 * If the ACL isn't being read yet, set our sentinel.  Otherwise, the
	 * current value of the ACL will not be ACL_NOT_CACHED and so our own
	 * sentinel will not be set; another task will update the cache.  We
	 * could wait for that other task to complete its job, but it's easier
	 * to just call ->get_acl to fetch the ACL ourself.  (This is going to
	 * be an unlikely race.)
	 */

Which presumes the only reason the acl could be anything other
ACL_NOT_CACHED is because get_acl() is already being called upon it in
another task.

I wanted something to mention ACL_DONT_CACHED so someone would at least
think about that case if they ever step up to modify the code.

The code is perfectly clear, the comment is not.   That scares me.

And I had to read the code about a dozen times before I realized the
ACL_DONT_CACHED case even exists.   Not useful when I am need to use
that to preserve historical fuse semantics.

So something is missing here even if my wording does not improve things.

Then we get this comment:
	/*
	 * 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.
	 */

Which was added in b8a7a3a66747 ("posix_acl: Inode acl caching fixes")
That comment is and always has been rubbish.

I don't have a clue what it is trying to say but it is not something
a person can use to write filesystem code with.

Truths:
- forget_cached_acl(inode, type) can be used to invalidate the acl
  cache.

- Calling forget_cached_acl from within the filesystems ->get_acl
  method won't prevent a cached value from being returend because
  ->get_acl will be set.

- Calling forget_cached_acl from within the filesystems ->get_acl
  method won't prevent a returned value from being cached
  because it the caching happens after ->get_acl returns.

- Setting inode->i_acl = ACL_DONT_CACHE is the only way to prevent
  a value from ->get_acl from being cached.

In summary I only care about two things.
1) ACL_NOT_CACHED being mentioned somewhere in get_acl so people looking
   at the code, and people updating the code will have a hint that they
   need to consider that case.

2) That misleading completely bogus comment being removed/fixed.

And yes I agree the code is clear.  The comments are not.

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.
         *
         * If the filesystem doesn't have a get_acl() function at all, we'll
         * just create the negative cache entry.

Eric