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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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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