linux-kernel - Re: [PATCH 1/5] Use mutex instead of semaphore in the Host AP driver

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Date:	Mon, 30 Jul 2007 12:23:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michael Buesch <mb@...sch.de>
Cc:	Satyam Sharma <satyam@...radead.org>,
	Matthias Kaehlcke <matthias.kaehlcke@...il.com>,
	linville@...driver.com, linux-wireless@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] Use mutex instead of semaphore in the Host AP
 driver

On Mon, 30 Jul 2007 19:09:38 +0200
Michael Buesch <mb@...sch.de> wrote:

> On Monday 30 July 2007, Satyam Sharma wrote:
> > 
> > On Mon, 30 Jul 2007, Michael Buesch wrote:
> > 
> > > On Sunday 29 July 2007 23:34, Matthias Kaehlcke wrote:
> > > > The Host AP driver uses a semaphore as mutex. Use the mutex API
> > > > instead of the (binary) semaphore.
> > > > 
> > > > Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@...il.com>
> > 
> > [ Something seems to have gone wrong with your diff / patch / script.
> >   There was no diff header here, which should have been. ]
> > 
> > > > -	res = down_interruptible(&local->rid_bap_sem);
> > > > +	res = mutex_lock_interruptible(&local->rid_bap_mtx);
> > > >  	if (res)
> > > >  		return res;
> > > >  
> > > > @@ -902,7 +902,7 @@ static int hfa384x_set_rid(struct net_device *dev, u16 rid, void *buf, int len)
> > > >  	/* RID len in words and +1 for rec.rid */
> > > >  	rec.len = cpu_to_le16(len / 2 + len % 2 + 1);
> > > >  
> > > > -	res = down_interruptible(&local->rid_bap_sem);
> > > > +	res = mutex_lock_interruptible(&local->rid_bap_mtx);
> > > >  	if (res)
> > > >  		return res;
> > > >  
> > > 
> > > Is res returned to userspace? If yes, that's not right.
> > 
> > Yup, that's not right.
> > 
> > > On a interrupted mutex allocation you should return
> > > -ERESTARTSYS to userspace.
> > 
> > Nope, userspace must not see ERESTARTSYS (I /think/ this is the third
> > time I'm participating in this exact same discussion :-)
> > 
> > If the return would be caught by a previous in-kernel caller in the
> > call chain, ERESTARTSYS is okay and it could try to restart the
> > operation. However, if the return goes unfiltered directly to
> > userspace, EINTR is the correct choice.
> 
> Last time I submitted a patch which returned EINTR to userspace,
> people came and said it was wrong. It was said that we must
> return ERESTARTSYS to the libc and the libc will convert that
> into either EINTR or automatically restart it immediately.
> (You can set that in the sigaction stuff).
> 
> So either the one who told me that last time was wrong, or you. :)
> 
> Andrew? I think you were involved in this discussion I mentioned.
> (It was about the HW-RNG chardev, but that doesn't matter here).

who, me?  signals?  Danger.

yes, I think ERESTARTSYS is correct.  That gets converted to -EINTR by the
arch's signal handling code (not by glibc) just before we return to
userspace.

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