rwlocks, correctness over speed.

[Robert Watson]

On Sat, 24 Nov 2007, Attilio Rao wrote:

>> I must have missed recursion arriving then -- I'll modify uipc_usrreq.c to 
>> set the recursion flag on the rwlock in UNIX domain sockets rather than 
>> doing the nasty hack that was previously required.  At the time, the hack 
>> was added because it seemed recursion was not going to be added to rwlocks, 
>> but sonewconn() behavior for listen sockets really ended up requiring it.
>
> attilio     2007-06-26 21:31:56 UTC
>
> FreeBSD src repository
>
> Modified files:
>   sys/kern             kern_rwlock.c
>   sys/sys              _rwlock.h rwlock.h
> Log:
> Introduce a new rwlocks initialization function: rw_init_flags.
> This is very similar to sx_init_flags: it initializes the rwlock using
> special flags passed as third argument (RW_DUPOK, RW_NOPROFILE,
> RW_NOWITNESS, RW_QUIET, RW_RECURSE).
> Among these, the most important new feature is probabilly that rwlocks
> can be acquired recursively now (for both shared and exclusive paths).

Yes, that was four months after I added rw_wowned(9) to work around the lack 
of recursion support. :-)  However, it looks like the man page was never 
updated?  It contains the following rather explicit language:

      Another important property is that shared holders of rwlock can recurse,
      but exclusive locks are not allowed to recurse.

Robert N M Watson
Computer Laboratory
University of Cambridge