RFC: dma_resv_unlock() and ww_acquire_ctx scope

[Daniel Vetter]

On Thu, Jan 19, 2023 at 11:37:39AM +0100, Maarten Lankhorst wrote:
> 
> On 2023-01-19 11:24, Paul Cercueil wrote:
> > 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.
> 
> Hey,
> 
> This example code will fail eventually. If you have DEBUG_LOCK_ALLOC
> enabled, a fake lock is inited in ww_acquire_init. If you don't free it
> using ww_acquire_fini(), lockdep will see that you free a live lock that was
> never released. PROVE_LOCKING will also complain that you never unlocked the
> ctx fake lock.
> 
> If you do call ww_acquire_fini, you will get a loud complain if you never
> released all locks, because ctx->acquired is non-zero.
> 
> Try with PROVE_LOCKING, your example will receive a lockdep splat. :)

Also CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y any time you change ww code please.
Otherwise it's just not fully tested.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch