linux-kernel - Re: [PATCH 3/4] fs/dcache: Avoid the try_lock loop in d

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Message-ID: <20180216173033.GJ25201@hirez.programming.kicks-ass.net>
Date:   Fri, 16 Feb 2018 18:30:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/4] fs/dcache: Avoid the try_lock loop in d_delete()

On Fri, Feb 16, 2018 at 04:09:32PM +0100, John Ogness wrote:
> 
>    inode = dentry->d_inode;
>    rcu_read_lock();         <- Protects d_inode from being freed,
>                                i.e. dentry->d_inode is a valid pointer
>                                even after dentry->d_lock is dropped
>    unlock(dentry->d_lock);
>    lock(inode->i_lock);
>    lock(dentry->d_lock);
>    rcu_read_unlock();

So that is entirely tricky, typically we have to have a lookup _after_
rcu_read_lock().

Here, we rely on not being able to call dentry_free() while we hold
d_lock, which ensure dentry must be valid in the freshly started
rcu-section.

And I suppose that same ensures dentry->d_ionde stays alive. But this
needs a comment at least.