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

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Message-ID: <CANN689EKsSZpq-oVPL3_y+ReT+vbz5wPbOs8iz8YzS_Wo-FboQ@mail.gmail.com>
Date:	Tue, 5 Mar 2013 08:32:14 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Oleg Nesterov <oleg@...hat.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 Tue, Mar 5, 2013 at 7:54 AM, Lai Jiangshan <laijs@...fujitsu.com> 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 (and I think this is actually the most likely way for it to happen).

We do need this to work correctly, but I don't expect we need it to be fast.
(could be wrong, this is only my intuition)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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