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Re: Inode Lock Scalability V7 (was V6)

From:  Al Viro <viro-AT-ZenIV.linux.org.uk>
To:  Nick Piggin <npiggin-AT-kernel.dk>
Subject:  Re: Inode Lock Scalability V7 (was V6)
Date:  Fri, 22 Oct 2010 04:07:28 +0100
Message-ID:  <20101022030728.GH19804@ZenIV.linux.org.uk>
Cc:  Dave Chinner <david-AT-fromorbit.com>, linux-fsdevel-AT-vger.kernel.org, linux-kernel-AT-vger.kernel.org
Archive-link:  Article

On Fri, Oct 22, 2010 at 01:34:44PM +1100, Nick Piggin wrote:

> > 	* walkers of the sb, wb and hash lists can grab ->i_lock at will;
> > it nests inside their locks.
> 
> What about if it is going on or off multiple data structures while
> the inode is live, like inode_lock can protect today. Such as putting
> it on the hash and sb list.

Look at the code.  You are overengineering it.  We do *not* need a framework
for messing with these lists in arbitrary ways.  Where would we need to
do that to an inode we don't hold a reference to or had placed I_FREEING
on and would need i_lock held by caller?  Even assuming that we need to
keep [present in hash, present on sb list] in sync (which I seriously doubt),
we can bloody well grab both locks before i_lock.

> > inodes.  It's not an accidental subtle property of the code, it's bloody
> > fundamental.
> 
> I didn't miss that, and I agree that at the point of my initial lock
> break up, the locking is "wrong". Whether you correct it by changing
> the lock ordering or by using RCU to do lookups is something I want to
> debate further.
> 
> I think it is natural to be able to lock the inode and have it lock the
> icache state.

Code outside of fs/inode.c and fs/fs-writeback.c generally has no business
looking at the full icache state, period.


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