linux-kernel - Re: linux-next: manual merge of the akpm tree with Linus' tree

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Message-ID: <CA+55aFxz2r3voP=W0oCns5MzMqBaeYxMLxp5AxRxjY23K3nvuA@mail.gmail.com>
Date:	Thu, 12 Sep 2013 17:56:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next <linux-next@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Chinner <dchinner@...hat.com>,
	Glauber Costa <glommer@...nvz.org>
Subject: Re: linux-next: manual merge of the akpm tree with Linus' tree

On Tue, Sep 10, 2013 at 4:37 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> From a quick look, this looks pretty broken:
>
>     if (list_lru_add(&dentry->d_sb->s_dentry_lru, &dentry->d_lru))
>         this_cpu_inc(nr_dentry_unused);
>     dentry->d_flags |= DCACHE_LRU_LIST;
>
> because if that list_lru_add() can fail, then we shouldn't set the
> DCACHE_LRU_LIST bit either.
>
> That said, I don't see how it can fail. We only do this with the
> dentry locked, and when it's not already on the LRU list. So I think
> the "if()" is just misleading and unnecessary - but the code works.

So I thought you'd clean this up. Looking again, it still seems really
confused, and I'm finding actual bugs.

You don't clear the DCACHE_LRU_LIST when you remove dentries from the
d_lru list. In other cases (like shrink_dentry_list), you clear just
the DCACHE_SHRINK_LIST.

As a result, the "if ()" isn't necessarily unnecessary, but there are
actual bugs. It looks like the dentry can be removed from the d_lru
lists without the bit ever getting cleared, and if that happens, it
will never be moved back.

The rule for DCACHE_LRU_LIST was - and should be - that the bit is set
IFF the d_lru list is not empty. So it gets set when a dentry is moved
to the LRU lists, but it _stays_ set if the dentry is moved to the
shrink_list. It then gets cleared when the dentry is removed from any
d_lru list (ie "list_del_init()").

I'll walk through the code, it looked suspicious. Maybe there's
something subtle that makes it work, but I don't see it.

           Linus
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