rwlocks, correctness over speed.

[Attilio Rao]

2007/11/24, Robert Watson <rwatson at freebsd.org>:
> On Sat, 24 Nov 2007, Attilio Rao wrote:
>
> > 2007/11/24, Robert Watson <rwatson at freebsd.org>:
> >
> >> While I'm no great fan of recursion, the reality is that many of our kernel
> >> subsystems are not yet ready to disallow recursion on locks.  Take a look
> >> at the cases where we explicitly enable recursive acquisition for
> >> mutexes--in practice, most network stack mutexes are recursive due to the
> >> recursive calling in the network stack.  While someday I'd like to think
> >> we'll be able to eliminate some of that, but it won't be soon since it
> >> requires significant reworking of very complicated code.  The current model
> >> in which recursion is explicitly enabled only where still required seems to
> >> work pretty well for the existing code, although it's hard to say yet in
> >> the code I've looked at whether read recursion would be required--the
> >> situations I have in mind would require purely write recursion.  There's
> >> one case in the UNIX domain socket code where we do a locked test and
> >> conditional lock/unlock with an rwlock for exclusive locking because
> >> recursion isn't currently supported, and that's not a usage I'd like to
> >> encourage more of.
> >
> > Oh, I just didn't notice this -- rwlock are only present in 7.0 and in 7.0
> > they support recursion in exclusive mode, so I'm not sure what do you mean
> > with 'recursion isn't currently supported'.
>
> 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).


Thanks,
Attilio


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