netdev - Re: [PATCH] lglock: add read-preference local-global rwlock

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Message-ID: <20130305163522.GA4179@redhat.com>
Date:	Tue, 5 Mar 2013 17:35:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Lai Jiangshan <eag0628@...il.com>, linux-doc@...r.kernel.org,
	peterz@...radead.org, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, namhyung@...nel.org,
	mingo@...nel.org, linux-arch@...r.kernel.org,
	linux@....linux.org.uk, xiaoguangrong@...ux.vnet.ibm.com,
	wangyun@...ux.vnet.ibm.com, paulmck@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	rusty@...tcorp.com.au, rostedt@...dmis.org, rjw@...k.pl,
	vincent.guittot@...aro.org, tglx@...utronix.de,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	sbw@....edu, tj@...nel.org, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] lglock: add read-preference local-global rwlock

On 03/05, Lai Jiangshan wrote:
>
> On 03/03/13 01:06, Oleg Nesterov wrote:
> > On 03/02, Michel Lespinasse wrote:
> >>
> >> My version would be slower if it needs to take the
> >> slow path in a reentrant way, but I'm not sure it matters either :)
> >
> > I'd say, this doesn't matter at all, simply because this can only happen
> > if we race with the active writer.
>
> It can also happen when interrupted. (still very rarely)
>
> arch_spin_trylock()
> 	------->interrupted,
> 		__this_cpu_read() returns 0.
> 		arch_spin_trylock() fails
> 		slowpath, any nested will be slowpath too.
> 		...
> 		..._read_unlock()
> 	<-------interrupt
> __this_cpu_inc()
> ....

Yes sure. Or it can take the local lock after we already take the global
fallback_lock.

But the same can happen with FALLBACK_BASE, just because we need to take
a lock (local or global) first, then increment the counter.

> (I worries to much. I tend to remove FALLBACK_BASE now, we should
> add it only after we proved we needed it, this part is not proved)

Agreed, great ;)

Oleg.

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