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, 4 insertions, 1 deletions

diff --git a/drivers/video/matrox/matroxfb_crtc2.c b/drivers/video/matrox/matroxfb_crtc2.c
index 7ac4c5f6145d..909e10a11898 100644
--- a/drivers/video/matrox/matroxfb_crtc2.c
+++ b/drivers/video/matrox/matroxfb_crtc2.c
@@ -289,13 +289,16 @@ static int matroxfb_dh_release(struct fb_info* info, int user) {
 #undef m2info
 }
 
-static void matroxfb_dh_init_fix(struct matroxfb_dh_fb_info *m2info) {
+static void matroxfb_dh_init_fix(struct matroxfb_dh_fb_info *m2info)
+{
         struct fb_fix_screeninfo *fix = &m2info->fbcon.fix;
 
         strcpy(fix->id, "MATROX DH");
 
+        mutex_lock(&m2info->fbcon.mm_lock);
         fix->smem_start = m2info->video.base;
         fix->smem_len = m2info->video.len_usable;
+        mutex_unlock(&m2info->fbcon.mm_lock);
         fix->ypanstep = 1;
         fix->ywrapstep = 0;
         fix->xpanstep = 8;      /* TBD */