linux-kernel - Re: [Fwd: [PATCH 2/7] lock_list: a fine grain locked double linked list]

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Date:	Sun, 28 Jan 2007 17:20:27 +0000
From:	Christoph Hellwig <hch@...radead.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [Fwd: [PATCH 2/7] lock_list: a fine grain locked double linked list]

> Provide a simple fine grain locked double link list.
> 
> It is build upon the regular double linked list primitives, spinlocks and RCU.
> 
> Locking is peculiar in that edges are locked, this avoid the circular lock
> dependancy created by the fact that the regular linked lists are circular.
> 
> Item deletion requires that both surrounding elements are locked, however since
> the locking rules dictate that we lock elements in a single direction we have
> to lock the previous element while it might be deleted under us. Hence the
> requirement that all elements are RCU freed.

I think implicitly locked data structures are very bad for code readability
and debugability.  What's even worse here is that we have a requirement that
all members are RCU freed.

Note that we also have another implicitly locked (and refcounted) list
implementation in klist.[ch] - if we find consensus that we want implicitly
locked list we should figure out whether we want lock_list or klist semantics
and stick to one of them.

What uses do you have planned for this data structure?  In general I think
we'd be better off to simplify the data structures as in my files_list_lock
proposal instead of complicating the locking.

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