V4L/DVB (7201): cx88-mpeg: Fix race condition in variable access

There was a possible race condition in the increment/decrement of
the active device references counter.
Thanks to Trent Piepho (xyzzy@speakeasy.org) for bringing it up.

Signed-off-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

diff --git a/drivers/media/video/cx88/cx88-mpeg.c b/drivers/media/video/cx88/cx88-mpeg.c
index f12733d..e357f41 100644 (file)
--- a/drivers/media/video/cx88/cx88-mpeg.c
+++ b/drivers/media/video/cx88/cx88-mpeg.c
@@ -615,8 +615,8 @@ static int cx8802_request_acquire(struct cx8802_driver *drv)
 
        if (drv->advise_acquire)
        {
-               core->active_ref++;
                mutex_lock(&drv->core->lock);
+               core->active_ref++;
                if (core->active_type_id == CX88_BOARD_NONE) {
                        core->active_type_id = drv->type_id;
                        drv->advise_acquire(drv);
@@ -634,14 +634,14 @@ static int cx8802_request_release(struct cx8802_driver *drv)
 {
        struct cx88_core *core = drv->core;
 
+       mutex_lock(&drv->core->lock);
        if (drv->advise_release && --core->active_ref == 0)
        {
-               mutex_lock(&drv->core->lock);
                drv->advise_release(drv);
                core->active_type_id = CX88_BOARD_NONE;
-               mutex_unlock(&drv->core->lock);
                mpeg_dbg(1,"%s() Post release GPIO=%x\n", __FUNCTION__, cx_read(MO_GP0_IO));
        }
+       mutex_unlock(&drv->core->lock);
 
        return 0;
 }