linux-kernel - Re: [PATCH][resubmit] TPM: update char dev BKL pushdown

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Message-Id: <1219847535.4768.18.camel@blackbox>
Date:	Wed, 27 Aug 2008 11:32:15 -0300
From:	Rajiv Andrade <srajiv@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	zohar@...ibm.com, dvelarde@...ibm.com, safford@...ibm.com
Subject: Re: [PATCH][resubmit] TPM: update char dev BKL pushdown

It was all about this section:

>       chip->data_buffer = kmalloc(TPM_BUFSIZE * sizeof(u8),
GFP_KERNEL);
>       if (chip->data_buffer == NULL) {
> -             chip->num_opens--;
> +             atomic_set(&chip->is_open, 0);
>               put_device(chip->dev);

num_opens wasn't protected by driver_lock, so we made num_opens/is_open
atomic_t. But an int seems too much for just a flag (as Serge pointed),
and the code would be cleaner if we make only this line atomic, by using
test_and_set_bit(). Thanks Alan.
I'll rewrite it.

Rajiv


On Tue, 2008-08-26 at 22:19 -0500, Serge E. Hallyn wrote:
> Quoting Alan Cox (alan@...rguk.ukuu.org.uk):
> > > +	atomic_set(&chip->is_open, 1);
> > > +	get_device(chip->dev); /* protect from chip disappearing */
> > 
> > Why not just use test_and_set_bit() ? You seem to be abusing atomic_t to
> > achieve this.
> 
> Good point.  Or heck just make it a simple flag.  Earlier I thought there
> was a place where driver_lock was taken just to do num_opens--, and so
> replacing the int num_opens with an atomic_t seemed worthwhile.  But since
> is_open is a boolean and now seems to be always protected by driver_lock,
> a flag seems best.
> 
> -serge
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