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

Michel Lespinasse walken at google.com
Wed Mar 6 03:32:14 EST 2013

On Tue, Mar 5, 2013 at 7:54 AM, Lai Jiangshan <laijs at cn.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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