Re: NPTL mutex and the scheduling priority

Sébastien Dugué
2006-06-13T11:59:46Z

On Tue, 2006-06-13 at 04:48 -0400, Jakub Jelinek wrote:
> On Mon, Jun 12, 2006 at 05:24:28PM +0200, S?bastien Dugu? wrote:
> > The patch you refer to is at
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=114725326712391&w=2
> >
> > But maybe a better solution for condvars would be to implement
> > something like a futex_requeue_pi() to handle the broadcast and
> > only use PI futexes all along in glibc.
>
> FUTEX_REQUEUE certainly should be able to requeue from normal futex
> to a PI futex or vice versa, I don't think it is desirable to create
> a separate futex cmds for that.

Indeed, that would be preferable but might get tricky.

> Now not sure what do you mean by "use PI futexes all along in glibc",
> certainly you don't mean using them for normal mutexes, right?
> FUTEX_LOCK_PI has effects the normal futexes shouldn't have.
> The condvars can be also used with PP mutexes and using PI for the cv
> internal lock unconditionally wouldn't be the right thing either.

I effectively meant using a PI futex for the cv __data.__futex but now
I realize it's a Really Bad Idea.

To summarize (correct me if I'm wrong), we need a way in the broadcast
case to promote the cv __data.__futex type to the type of the external
mutex (PI, PP, normal) in the requeue path. Therefore we need the
ability to requeue waiters on a regular futex onto a PI futex.

Ingo, Thomas, is this feasible?

Sébastien.