Re: [Threads-devel] RFC: 1st pthread shared_mutex refactoring patch

Subject: Re: [Threads-devel] RFC: 1st pthread shared_mutex refactoring patch
From: Fredrik Orderud (forderud_at_[hidden])
Date: 2013-03-31 16:20:12

On Tue, Mar 26, 2013 at 6:06 AM, Vicente J. Botet Escriba <
vicente.botet_at_[hidden]> wrote:

>  Le 24/03/13 18:24, Vicente J. Botet Escriba a écrit :
>
> Le 18/03/13 00:06, Fredrik Orderud a écrit :
>
>  v2/shared_mutex indeed appears to be quite different. From what I can
> see, it lacks the exclusive_waiting_blocked field. This makes me question
> if it is capable of prioritizing write-lock request in favor of read-lock
> requests, as the pthread and win32 equivalents are doing. What are your
> thoughts on this?
>
> I suspect that Howard Hinnant prefered a fair solution.
>
> After more deep insight I'm wondering if write_entered_ is not used to
> ensure priority for writers. Note that write_entered_ doesn't mean that the
> mutex is in exclusive mode but that some one has requested to be in.  The
> mutex is in exclusive mode when write_entered_ and n_readers_==0.
>
> What do you think?
>

It appears like you have a point there Vicente. v2/shared_mutex does indeed
seem to prioritize exclusive mode lock attempts through write_entered_.
Sorry for not noticing that earlier.

I think it would be easier to understand v2 shared_mutex & upgrade_mutex if
the implementation had been "cleaned up" a bit. Especially, the gate1_ &
gate2_ can probably be given more informative names. Also, replacing all
the bitmask-logic with bitfields would make the code easier to read.

Regards,
Fredrik