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Date: Wed, 30 Apr 2014 17:18:23 -0700
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Subject: Re: dcache shrink list corruption?
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Miklos Szeredi <miklos@szeredi.hu>, Dave Chinner <david@fromorbit.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Wed, Apr 30, 2014 at 4:43 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> OK, done and force-pushed.  Should propagate in a few...

That made it more obvious how the DCACHE_MAY_FREE case ends up
working. And in particular, mind rewriting this:

    if (dentry->d_flags & DCACHE_MAY_FREE) {
        spin_unlock(&dentry->d_lock);
        dentry_free(dentry);
    } else {
        spin_unlock(&dentry->d_lock);
    }
    return parent;

as just

    bool free = dentry->d_flags & DCACHE_MAY_FREE;
    spin_unlock(&dentry->d_lock);
    if (free)
        dentry_free(dentry);
    return parent;

instead? In fact, I get the feeling that the other case later on
really fits the same model:

    spin_lock(&dentry->d_lock);
    if (dentry->d_flags & DCACHE_SHRINK_LIST) {
        dentry->d_flags |= DCACHE_MAY_FREE;
        spin_unlock(&dentry->d_lock);
    } else {
        spin_unlock(&dentry->d_lock);
        dentry_free(dentry);
  }

ends up really being better as

    spin_lock(&dentry->d_lock);
    free = 1;
    if (dentry->d_flags & DCACHE_SHRINK_LIST) {
        dentry->d_flags |= DCACHE_MAY_FREE;
        free = 0;
    }
    spin_unlock(&dentry->d_lock);
    if (free)
        dentry_free(dentry);
    return parent;

and then suddenly it looks like we have a common exit sequence from
that dentry_kill() function, no?

(The earlier "unlock_on_failure" exit case is altogether a different case).

I dunno. Maybe not a big deal, but one reason I prefer doing that
"free" flag is because I really tend to prefer the simple case of
lock-unlock pairing cleanly at the same level. NOT the pattern where
you have one lock at one indentation level, paired with multiple
unlocks for all the different cases.

                  Linus