linux-kernel - Re: [Linux-fbdev-devel] [PATCH] fb: Remove use of lock_kernel / unlock

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Message-Id: <20080415221707.82da3d42.akpm@linux-foundation.org>
Date:	Tue, 15 Apr 2008 22:17:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Thiago Galesi <thiagogalesi@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-fbdev-devel@...ts.sourceforge.net
Subject: Re: [Linux-fbdev-devel] [PATCH] fb: Remove use of lock_kernel /
 unlock_kernel in fbmem

On Sun, 13 Apr 2008 11:50:07 -0300 Thiago Galesi <thiagogalesi@...il.com> wrote:

> This patch removes lock_kernel(), unlock_kernel() usage in fbmem.c and replaces it with a mutex

It isn't that simple, alas.

vfs_ioctl() runs lock_kernel() prior to calling fb_ioctl(), so the
lock_kernel()s in fb_compat_ioctl() are actually providing exclusion against
fb_ioctl().  Your patch would break that.

A suitable fix might be to do

__fb_ioctl(...)
{
	<copy fb_ioctl() into here>
}

fb_ioctl(...)
{
	mutex_lock(&info->hwlock);
	__fb_ioctl(...);
	mutex_unlock(&info->hwlock);
}

and then change fb_compat_ioctl() to call __fb_ioctl().  All the other
callers of fb_ioctl() would need to be reviewed - see if they need to take
the mutex then call __fb_ioctl(), or they might be OK as they are, calling
fb_ioctl().

Then we can switch fb_fops over to

	.ioctl	= NULL,
	.unlocked_ioctl = fb_ioctl,


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