linux-kernel - Re: [PATCH 11/18] fs: Introduce per-bucket inode hash locks

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Message-ID: <20101019070057.GA7770@amd>
Date:	Tue, 19 Oct 2010 18:00:57 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Nick Piggin <npiggin@...nel.dk>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/18] fs: Introduce per-bucket inode hash locks

On Mon, Oct 18, 2010 at 12:21:05PM -0400, Christoph Hellwig wrote:
> On Mon, Oct 18, 2010 at 06:16:50PM +0200, Andi Kleen wrote:
> > > Hiding the type of lock, and hiding the fact that it sets the low bit?
> > > I don't agree. We don't have synchronization in our data structures,
> > > where possible, because it is just restrictive or goes wrong when people
> > > don't think enough about the locking.
> > 
> > I fully agree. The old skb lists in networking made this mistake
> > long ago and it was a big problem, until people essentially stopped 
> > using it (always using __ variants) and it was eventually removed.
> > 
> > Magic locking in data structures is usually a bad idea.
> 
> Err, there is no implicit locking in the calls to hlist_*.  There
> is just two small wrappers for the bit-lock/unlock so that the callers
> don't have to know how the lock is overloaded onto the pointer in the
> list head.  

But it is still "magic". Because you don't even know whether it
is a spin or sleeping lock, let alone whether it is irq or bh safe.
You get far more information seeing a bit_spin_lock(0, &hlist) call
than hlist_lock().

Even if you do rename them to hlist_bit_spin_lock, etc. Then you need
to add variants for each type of locking a caller wants to do on it.
Ask Linus what he thinks about that.

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