data_request: for prefetches, only take a reference on the destination replicate when really transferring

For fetches, we want to make sure to keep the room as soon as the fetch
request, to avoid interlocks. For prefetches however, we want to allow
buffer eviction to proceed, since we may get processed way later.

src/datawizard/data_request.c

@@ -194,8 +194,9 @@ struct _starpu_data_request *_starpu_create_data_request(starpu_data_handle_t ha
 
 	_starpu_spin_lock(&r->lock);
 
-	/* Take a reference on the target for the request to be able to write it */
-	if (dst_replicate)
+	/* For a fetch, take a reference as soon as now on the target, to avoid
+	 * replicate eviction */
+	if (is_prefetch == STARPU_FETCH && dst_replicate)
 		dst_replicate->refcnt++;
 	handle->busy_count++;
 
@@ -525,11 +526,21 @@ static int starpu_handle_data_request(struct _starpu_data_request *r, unsigned m
 	struct _starpu_data_replicate *dst_replicate = r->dst_replicate;
 
 	enum starpu_data_access_mode r_mode = r->mode;
+	unsigned added_ref = 0;
 
 	STARPU_ASSERT(!(r_mode & STARPU_R) || src_replicate);
 	STARPU_ASSERT(!(r_mode & STARPU_R) || src_replicate->allocated);
 	STARPU_ASSERT(!(r_mode & STARPU_R) || src_replicate->refcnt);
 
+	/* For prefetches, we take a reference on the destination only now that
+	 * we will really try to fetch the data (instead of in
+	 * _starpu_create_data_request) */
+	if (r->prefetch > STARPU_FETCH)
+	{
+		added_ref = 1;	/* Note: we might get upgraded while trying to allocate */
+		dst_replicate->refcnt++;
+	}
+
 	_starpu_spin_unlock(&r->lock);
 
 	/* FIXME: the request may get upgraded from here to freeing it... */
@@ -549,6 +560,11 @@ static int starpu_handle_data_request(struct _starpu_data_request *r, unsigned m
 	{
 		/* If there was not enough memory, we will try to redo the
 		 * request later. */
+
+		if (added_ref)
+			/* Drop ref until next try */
+			dst_replicate->refcnt--;
+
 		_starpu_spin_unlock(&handle->header_lock);
 		return -ENOMEM;
 	}
@@ -882,7 +898,13 @@ int _starpu_check_that_no_data_request_is_pending(unsigned node, unsigned peer_n
 
 void _starpu_update_prefetch_status(struct _starpu_data_request *r, enum starpu_is_prefetch prefetch)
 {
+	_starpu_spin_checklocked(&r->handle->header_lock);
 	STARPU_ASSERT(r->prefetch > prefetch);
+
+	if (prefetch == STARPU_FETCH)
+		/* That would have been done by _starpu_create_data_request */
+		r->dst_replicate->refcnt++;
+
 	r->prefetch=prefetch;
 
 	if (prefetch >= STARPU_IDLEFETCH)