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Date: Thu, 21 Oct 2004 13:11:17 +0200
From: Jens Axboe <axboe@suse.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Rui Nuno Capela <rncbc@rncbc.org>, Ingo Molnar <mingo@elte.hu>,
       LKML <linux-kernel@vger.kernel.org>, Lee Revell <rlrevell@joe-job.com>,
       mark_h_johnson@raytheon.com, "K.R. Foley" <kr@cybsft.com>,
       Bill Huey <bhuey@lnxw.com>, Adam Heath <doogie@debian.org>,
       Florian Schmidt <mista.tapas@gmx.net>,
       Michal Schmidt <xschmi00@stud.feec.vutbr.cz>,
       Fernando Pablo Lopez-Lezcano <nando@ccrma.stanford.edu>
Subject: Re: [patch] Real-Time Preemption, -RT-2.6.9-rc4-mm1-U8
Message-ID: <20041021111116.GE10531@suse.de>
References: <20041018145008.GA25707@elte.hu> <20041019124605.GA28896@elte.hu> <20041019180059.GA23113@elte.hu> <20041020094508.GA29080@elte.hu> <30690.195.245.190.93.1098349976.squirrel@195.245.190.93> <1098350190.26758.24.camel@thomas> <20041021095344.GA10531@suse.de> <1098352441.26758.30.camel@thomas> <20041021101103.GC10531@suse.de> <1098353505.26758.38.camel@thomas>
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On Thu, Oct 21 2004, Thomas Gleixner wrote:
> 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.

Semaphore and mutex has always been the same thing in Linux. Apparently
this isn't so in Ingos tree, you should make a clear distinction on
which you are discussing.

-- 
Jens Axboe

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