linux-kernel - Re: [PATCH] isicom: Fix buffer allocation

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Date:	Tue, 29 Apr 2008 11:59:17 -0400
From:	Alan Cox <alan@...hat.com>
To:	Olivier Galibert <galibert@...ox.com>,
	linux-kernel@...r.kernel.org, alan@...hat.com
Subject: Re: [PATCH] isicom: Fix buffer allocation

On Tue, Apr 29, 2008 at 05:26:07PM +0200, Olivier Galibert wrote:
> > @@ -813,15 +813,13 @@
> >  		return 0;
> >  	if (!port->xmit_buf) {
> >  		/* Relies on BKL */
> > +		unsigned long page  = get_zeroed_page(GFP_KERNEL);
> > +		if (page == 0)
> >  			return -ENOMEM;
> > +		if (port->xmit_buf)
> > +			free_page(page);
> > +		else
> > +			port->xmit_buf = (unsigned char *) page;
> >  	}
> 
> 
> Still looks rather strange.  An if(x) inside an if(!x) ?

The joys of parallelism. The serial drivers mostly do this because they
are using the BKL for open/close paths being a bit prehistoric (its on
the hit list ;))

CPU #1

	open
	port->xmit_buf == NULL
	get_zeroed_page [Can sleep dropping BKL]

CPU #2

	get_zeroed_page
	port->xmit_buf == NULL
		port->xmit_buf = page

CPU #1

	port->xmit_buf != NULL
		free page

A fine example of why proper locking is good ;)

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