linux-kernel - Re: [PATCH V3] powerpc: Implement {cmp}xchg for u8 and u16

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Message-ID: <20160425153710.GG3448@twins.programming.kicks-ass.net>
Date:	Mon, 25 Apr 2016 17:37:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
	boqun.feng@...il.com, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de
Subject: Re: [PATCH V3] powerpc: Implement {cmp}xchg for u8 and u16

On Mon, Apr 25, 2016 at 06:10:51PM +0800, Pan Xinhui wrote:
> > So I'm not actually _that_ familiar with the PPC LL/SC implementation;
> > but there are things a CPU can do to optimize these loops.
> > 
> > For example, a CPU might choose to not release the exclusive hold of the
> > line for a number of cycles, except when it passes SC or an interrupt
> > happens. This way there's a smaller chance the SC fails and inhibits
> > forward progress.

> I am not sure if there is such hardware optimization.

So I think the hardware must do _something_, otherwise competing cores
doing load-exlusive could life-lock a system, each one endlessly
breaking the exclusive ownership of the other and the store-conditional
always failing.

Of course, there are such implementations, and they tend to have to put
in explicit backoff loops; however, IIRC, PPC doesn't need that. (See
ARC for an example that needs to do this.)