Re: circular locking: /proc/sys/vm/drop_caches

[Andrew Morton <akpm@linux-foundation.org>]

> On Fri, 23 Feb 2007 07:35:38 +0100 Ingo Molnar <mingo@elte.hu> wrote:
> i just got the lockdep warning below when doing:
> 
>   echo 3 > /proc/sys/vm/drop_caches
> 
> is this a known quirk in the drop_caches code?

It is - we're taking inode_lock at just the wrong level there and I don't
know how to fix it.  The basic problem is that we're doing a list_head
walk: if we drop the lock we have to start again.

Well, I do know how to fix it: rip the various inode lists and use a tagged
radix tree instead - make writeback's per-superblock inode walk work the
same as writeback's per-inode page walk.  That will also alow us to drop
the lock and then resume the drop_caches thing at the point where we left
off.

But it's a large change and has some quite tricky issues which need to be
carried over from the existing code and I don't have time to do it.

The only (lame) thing I can say for now is that drop_caches is a
debug-only, root-only thing :(

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