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Date: Tue, 5 Mar 2013 17:35:22 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Lai Jiangshan <laijs@cn.fujitsu.com>
Subject: Re: [PATCH] lglock: add read-preference local-global rwlock
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Cc: Lai Jiangshan <eag0628@gmail.com>, linux-doc@vger.kernel.org,
 peterz@infradead.org, fweisbec@gmail.com,
 Michel Lespinasse <walken@google.com>, mingo@kernel.org,
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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.