linux-kernel - Re: [PATCH v2] powerpc ticket locks

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Message-ID: <1392089915.3996.60.camel@pasglop>
Date:	Tue, 11 Feb 2014 14:38:35 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Torsten Duwe <duwe@....de>, Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Scott Wood <scottwood@...escale.com>,
	Tom Musta <tommusta@...il.com>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] powerpc ticket locks

On Tue, 2014-02-11 at 02:56 +0000, Al Viro wrote:
> > So the question is, is it reasonable to have the ref smaller than
> > 32-bit...
> 
> Every time you open a file, you bump dentry refcount.  Something like
> libc or ld.so will be opened on just about every execve(), so I'd say
> that 16 bits is far too low.  If nothing else, 32 bits might be too
> low on 64bit boxen...

So back to square 1 ... we can't implement together lockref, ticket
locks, and our lock confer mechanism within 64-bit.

I see two options at this stage. Both require a custom implementation
of lockref for powerpc, so some ifdef's such that we can replace the
generic implementation completely.

 - We can use a small ref, and when it's too big, overflow into a larger
one, falling back to the "old style" lock + ref (an overflow bit or a
compare with ffff)

 - We can have lockref "build" it's own lock out of the ticketpair and
ref, keeping the owner in a separate word. The owner doesn't strictly
need to be atomic.

Both are gross though :(

Anybody has a better idea ?

Ben.


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