FYI:

Forwarding the response from Howard Hinnant to the mailing list.

On Apr 9, 2013, at 1:25 PM, Fredrik Orderud <forderud@gmail.com> wrote:

Howard,

Vicente J. Botet Escriba and I have recently discussed your usage of "bitmask" manipulation in boost::thread v2/shared_mutex & upgrade_mutex. In particular, I've proposed to replace the "bitmask" manipulaiton with equivalent "bitfields" for improved readability. You can read through the discussion through the link below.

Could you please give us your thoughts on the discusion? Are there e.g. any specific reason for not using bitfields?

Boost::thread discussion origin:
http://lists.boost.org/threads-devel/2013/03/0697.php


Thanks in advance,
Fredrik Orderud

Hi Fredrik, Vicente,

This looks like a good idea to me (putting in bitfields).  There may be places where I'm reading or writing more than one bitfield at a time, so watch out for that.  It has been so long since I wrote this code, I don't remember for sure.  Indeed, if I'm doing that, that may have been the motivation for not using bitfields.  However I imagine if there is any performance lost, it could be gained back by unioning the bitfield rep with the unsigned rep.

I can see that the two of you are doing a very careful review of this complicated code.  That is much appreciated.  I don't currently have the time for such a review, even though it is my own code.

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. 

My intent was to use a fair algorithm.

Contention for gate1 should be completely fair.  However once a writer gets past gate1, then he is prioritized as the next owner.  Same is true of a reader though:  Once past gate1 a reader is also the next owner.

Howard