On Fri, Sep 12, 2014 at 11:27:07AM +0100, Lionel Landwerlin wrote:
> When using Mesa and LibVA in the same process, one would like to be
> able bind buffers from the output of the decoder to a GL texture
> through an EGLImage.
>
> LibVA can reuse buffers allocated by Gbm through a file descriptor. It
> will then wrap it into a drm_intel_bo with
> drm_intel_bo_gem_create_from_prime().
>
> The problem at the moment is that both library get a different
> drm_intel_bufmgr object when they call drm_intel_bufmgr_gem_init()
> even though they're using the same drm file descriptor. As a result,
> instead of manipulating the same buffer object for a given file
> descriptor, they get 2 different drm_intel_bo objects and 2 different
> refcounts, leading one of the library to get errors from the kernel on
> invalid BO when one of the 2 library is done with a shared buffer.
>
> This patch modifies drm_intel_bufmgr_gem_init() so, given a file
> descriptor, it will look for an already existing drm_intel_bufmgr
> using the same file descriptor and return that object.
>
> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
So close.
> +static void
> +drm_intel_bufmgr_gem_unref(drm_intel_bufmgr *bufmgr)
> +{
> + drm_intel_bufmgr_gem *bufmgr_gem = (drm_intel_bufmgr_gem *)bufmgr;
> +
> + if (atomic_dec_and_test(&bufmgr_gem->refcount)) {
> + assert(pthread_mutex_lock(&bufmgr_list_mutex) == 0);
(Oh, don't use assert for expressions with side-effects)
> +
> + if (atomic_read(&bufmgr_gem->refcount)) {
As a thought exercise, now consider what happens if thread B grabs a
reference to this bufmgr and frees it all before this thread wakes up?
Double free, kaboom.
This does have to be an atomic_add_unless, which does not yet exist in
libdrm:
static inline int atomic_add_unless(atomic_t *v, int add, int unless)
{
int c, old;
c = atomic_read(v);
while (c != unless && (old = atomic_cmpxchg(v, c, c + add)) != c)
c = old;
return c == unless;
}
static void
drm_intel_bufmgr_gem_unref(drm_intel_bufmgr *bufmgr)
{
drm_intel_bufmgr_gem *bufmgr_gem = (drm_intel_bufmgr_gem *)bufmgr;
if (atomic_add_unless(&bufmgr_gem->refcount, -1, 1)) {
pthread_mutex_lock(&bufmgr_list_mutex);
if (atomic_dec_and_test(&bufmgr_gem->refcount)) {
free stuff;
}
pthread_mutex_unlock(&bufmgr_list_mutex);
}
}
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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