RFC: dma_resv_unlock() and ww_acquire_ctx scope

[Paul Cercueil]

Hi,

Just a reflexion I have after an intensive (and intense) debugging
session.

I had the following code:


int my_dma_resv_lock(struct dma_buf *dmabuf)
{
	struct ww_acquire_ctx ctx;
	int ret;

	ww_acquire_init(&ctx, &reservation_ww_class);

	ret = dma_resv_lock_interruptible(dmabuf->resv, &ctx);
	if (ret) {
		if (ret != -EDEADLK)
			return ret;

		ret = dma_resv_lock_slow_interruptible(dmabuf->resv,
						       &ctx);
	}

	return ret;
}


Then I would eventually unlock the dma_resv object in the caller
function. What made me think this was okay, was that the API itself
suggests it's okay - as dma_resv_unlock() does not take the
"ww_acquire_ctx" object as argument, my assumption was that after the
dma_resv was locked, the variable could go out of scope.

I wonder if it would be possible to change the API a little bit, so
that it is less prone to errors like this. Maybe by doing a struct copy
of the initialized ctx into the dma_resv object (if at all possible),
so that the original can actually go out of scope, or maybe having
dma_resv_unlock() take the ww_acquire_ctx as argument, even if it is
not actually used in the function body - just to make it obvious that
it is needed all the way to the point where the dma_resv is unlocked.

This email doesn't have to result in anything, I just thought I'd share
one point where I feel the API could be made less error-prone.

Cheers,
-Paul