linux-kernel - Re: [BUG] lock_parent() breakage when used from shrink_dentry_list() (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry

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Date:   Fri, 23 Feb 2018 13:35:52 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [BUG] lock_parent() breakage when used from shrink_dentry_list()
 (was Re: [PATCH v2 6/6] fs/dcache: Avoid remaining try_lock loop in shrink_dentry_list())

On Fri, Feb 23, 2018 at 12:13 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> Look:
>         dentry placed on a shrink list
>         we pick the fucker from the list and lock it.
>         we call lock_parent() on it.
>                 dentry is not a root and it's not deleted, so we proceed.
>                 trylock fails.
>                 we grab rcu_read_lock()
>                 we drop dentry->d_lock

[ deleted the bad things ]

Should we just instead get the ref to the dentry before dropping the
lock, so that nobody else can get to dentry_kill?

This is too subtle, and your fix to check d_lockref.count < 0 sounds
wrong to me. If it's really gone, maybe it has been reused and the
refcount is positive again, but it's something else than a dentry
entirely?

Hmm.

No, you extended the rcu read section, so I guess your patch is fine.
And lock_parent already has that pattern, soiit's not new.

Ok, I agree, looks like lock_parent should just re-check that thing
that it already checked earler, but that now might be true again
because of we dropped d_lock.

              Linus