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Subject: Re: [patch] Real-Time Preemption, -RT-2.6.9-rc4-mm1-U8
From: Thomas Gleixner <tglx@linutronix.de>
Reply-To: tglx@linutronix.de
To: Jens Axboe <axboe@suse.de>
Cc: Rui Nuno Capela <rncbc@rncbc.org>, Ingo Molnar <mingo@elte.hu>,
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Date: Thu, 21 Oct 2004 12:11:45 +0200
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On Thu, 2004-10-21 at 12:11, Jens Axboe wrote:
> 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).

Yeah, for a semaphore it is, but not for a mutex.

IMHO, this is not clearly seperated and therefor produces a lot of
confusion.

tglx


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