linux-kernel - Re: [rfc] "fair" rw spinlocks

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Message-ID: <87pr72529w.fsf@basil.nowhere.org>
Date:	Sat, 28 Nov 2009 12:15:07 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [rfc] "fair" rw spinlocks

"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:
>
> The usual trick would be to keep per-fair-rwlock state in per-CPU
> variables.  If it is forbidden to read-acquire one nestable fair rwlock
> while read-holding another, then this per-CPU state can be a single
> pointer and a nesting count.  On the other hand, if it is permitted to
> read-acquire one nestable fair rwlock while holding another, then one
> can use a small per-CPU array of pointer/count pairs.

The problem is that in preemptible kernels kernel threads can switch
CPUs all the time.  How would you sync the per CPU state then?

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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