linux-kernel - Re: [RFC PATCH] shrink_dcache

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Message-ID: <877h10957n.fsf@tucsk.pomaz.szeredi.hu>
Date:	Mon, 09 Jan 2012 18:05:32 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, mgorman@...e.de, gregkh@...e.de,
	hch@...radead.org, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH] shrink_dcache_parent() deadlock

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Mon, Jan 9, 2012 at 2:58 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
>>
>> This patch adds a new dentry flag that is set when the dentry is removed
>> from the lru and put on the list being processed by
>> shrink_dentry_list().  The flag is cleared in dentry_lru_del() which is
>> called if the dentry gets a new reference just before it's pruned.
>>
>> Thoughts?
>
> Looks reasonable. Were you actually able to reproduce the hang, or how
> did you notice?

We got a bug report from a partner about hang during reboot.  It's
actually quite easily reproducible with:

while true; do
        echo -bond0 > /sys/class/net/bonding_masters
        echo +bond0 > /sys/class/net/bonding_masters
        echo -bond1 > /sys/class/net/bonding_masters
        echo +bond1 > /sys/class/net/bonding_masters
done

That reliably triggers the soft lookup detector for several machines
that we tested.  It's rather timing sensitive though, because turning on
DEBUG_SPINLOCK makes it go away.

I had to add printks to see what's going on, because it wasn't obvious
from the stack traces and crash dumps.

Thanks,
Miklos
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