Re: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit reader rt locks

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 2012-05-15 at 19:31 +0200, Peter Zijlstra wrote:
> On Tue, 2012-05-15 at 13:25 -0400, Steven Rostedt wrote:
> > On Tue, 2012-05-15 at 11:42 -0400, Steven Rostedt wrote:
> > > On Tue, 2012-05-15 at 17:06 +0200, Peter Zijlstra wrote:
> > > > On Tue, 2012-05-15 at 10:03 -0400, Steven Rostedt wrote:
> > > > > 
> > > > > where readers may nest (the same task may grab the same rwsem for
> > > > > read multiple times), but only one task may hold the rwsem at any
> > > > > given
> > > > > time (for read or write).
> > > > 
> > > > Humm, that sounds iffy, rwsem isn't a recursive read lock only rwlock_t
> > > > is.
> > > 
> > > In that case, current -rt is broken. As it has it being a recursive lock
> > > (without my patch).
> 
> Nah not broken, just pointless. A recursive lock that's not used
> recursively is fine.

Heh, sure :-) But as -rt keeps it recursive, I didn't want to change
that.

> 
> > 
> > Why wouldn't it be recursive. If two different tasks are allowed to grab
> > a read lock at the same time, why can't the same task grab a read lock
> > twice? As long as it releases it the same amount of times.
> > 
> > Now you can't grab a read lock if you have the write lock.
> 
> rwsem is fifo-fair, if a writer comes in between the second read
> acquisition (even by the same task) would block and you'd be a deadlock
> since the write won't succeed since you're still holding a reader.

Yep agreed. And this patch didn't change that either.

-- Steve