fbdev: add mutex for fb_mmap locking

Add a mutex to avoid a circular locking problem between the mm layer
semaphore and fbdev ioctl mutex through the fb_mmap() call.

Also, add mutex to all places where smem_start and smem_len fields change
so the mutex inside the fb_mmap() is actually used.  Changing of these
fields before calling the framebuffer_register() are not mutexed.

This is 2.6.31 material.  It removes one lockdep (fb_mmap() and
register_framebuffer()) but there is still another one (fb_release() and
register_framebuffer()).  It also cleans up handling of the smem_start and
smem_len fields used by mutexed section of the fb_mmap().

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/video/w100fb.c b/drivers/video/w100fb.c
index d0674f1e3f10..8a141c2c637b 100644
--- a/drivers/video/w100fb.c
+++ b/drivers/video/w100fb.c
@@ -523,6 +523,7 @@ static int w100fb_set_par(struct fb_info *info)
 		info->fix.ywrapstep = 0;
 		info->fix.line_length = par->xres * BITS_PER_PIXEL / 8;
 
+		mutex_lock(&info->mm_lock);
 		if ((par->xres*par->yres*BITS_PER_PIXEL/8) > (MEM_INT_SIZE+1)) {
 			par->extmem_active = 1;
 			info->fix.smem_len = par->mach->mem->size+1;
@@ -530,6 +531,7 @@ static int w100fb_set_par(struct fb_info *info)
 			par->extmem_active = 0;
 			info->fix.smem_len = MEM_INT_SIZE+1;
 		}
+		mutex_unlock(&info->mm_lock);
 
 		w100fb_activate_var(par);
 	}