Re: [patch] Real-Time Preemption, -RT-2.6.9-rc4-mm1-U8

[Jens Axboe]

On Thu, Oct 21 2004, Thomas Gleixner wrote:
> On Thu, 2004-10-21 at 11:53, Jens Axboe wrote:
> > On Thu, Oct 21 2004, Thomas Gleixner wrote:
> > > On Thu, 2004-10-21 at 11:12, Rui Nuno Capela wrote:
> > > >  [<e018e139>] queuecommand+0x70/0x7c [usb_storage] (24)
> > > 
> > > As I already pointed out, this is a problem due to up(sema) in
> > > queuecommand. That's one of the semaphore abuse points, which needs to
> > > be fixed. 
> > > 
> > > The problem is that semaphores are hold by Process A and released by
> > > Process B, which makes Ingo's checks trigger
> > 
> > That's utter crap, it's perfectly valid use.
> 
> It's not!
> 
> >From the code:
> 
>          init_MUTEX_LOCKED(&(us->sema));
> 
> This is used to wait for command completion and therefor we have the
> completion API. It was used this way because the ancestor of completion
> (sleep_on) was racy !

I didn't look at the USB code, I'm just saying that it's perfectly valid
use of a semaphore the pattern you describe (process A holding it,
process B releasing it).

-- 
Jens Axboe

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