sem_post() performance

[John Baldwin]

On Wednesday, September 24, 2014 06:19:43 PM Konstantin Belousov wrote:
> On Wed, Sep 24, 2014 at 11:04:28AM -0400, John Baldwin wrote:
> > On Wednesday, September 24, 2014 05:45:19 PM Konstantin Belousov wrote:
> > > I think it is even worse now. If the application linked against FBSD_1.0
> > > (ksem) semaphores implementation runs on the modern host, it cannot
> > > share semaphore with modern binary.
> > 
> > Correct.  We changed sem_t's ABI hence the compat shims IIRC.
> 
> I mean, that new and old semaphores cannot IPC.

Yes.

> > > Since this is not considered significant problem, we can avoid compat
> > > code there as well.
> 
> By compat code I mean the switch on SEM_VERSION, not symversioning the
> libc symbols.

Ah, ok.  My idea was we can always add that back in later if it becomes an 
issue (though it would only solve half the problem).

> > I think if we leave sem_t alone there is no reason to create new compat
> > shims for this change, but the existing FBSD_1.0 versions have to remain
> > for people using old binaries, yes?
> 
> Assume we applied new symver to semaphores which use new binary protocol
> on the existing sem_t, and, to make it reasonable, use old protocol
> when sem_t is accessed by older functions.  Then, old binaries cannot
> IPC with new binaries, although both are dynamically linked to libc.
> IMO this is worse than the problem of different libc versions not
> communicating.

Yes, I don't think we should bump symver for this change, just change 
SEM_MAGIC from 'sem1' to 'sem2'.

-- 
John Baldwin