Hi,
On 23 Feb 2022, at 17:01, Stefan Dösinger <stefan(a)codeweavers.com> wrote:
- while (InterlockedCompareExchange(&resource->access_count, 0, 0)) + /* A resource is considered busy between queueing a command that reads it and the execution of that + * command. We use the head and tail pointer of the default CS queue for tracking the access time. + * We can't track commands on the map queue this way. If a map command is handled asynchronously the + * resource fencing needs to be handled some other way. + * + * The queue head and tail will wrap around when the 32 bit ULONG is exhausted. We therefore need to + * handle a few cases: + * + * A...access_time in the resource + * H...queue write head + * T...queue read tail + * + * Case 1: + * |.....T------A-----H..........| + * The resource is busy because the access time is between head and tail. No wrap-around has happened. + * + * Case 2: + * |..A.....T---------H..........| + * The resource is idle, the last command using it has been executed. + * + * Case 3: + * |........T---------H.....A....| + * The resource is idle, the last command using it has been executed and the head and tail have since + * wrapped around. + * + * Case 4: + * |--A---H.................T----| + * Resource is busy, HEAD has wrapped around, tail not yet. Note that Head < Tail + * + * Case 5: + * |------H....A............T----| + * Resource is idle. Head has wrapped around, tail not yet. + * + * Case 6: + * A + * T + * |........H....................| + *
There’s a case 7, |--H.................T--A--|, which I don’t think is handled correctly (it goes to the case 5 path). I think it could handled uniformly with a slightly simpler condition like greater_wrap(A,T) && greater_wrap(H,A) => busy, where greater_wrap(x, y) = (x - y) < UINT_MAX/2. That assumes that head and tail are close together relative to the range size, but it seems reasonable to me.
+ * Queue is empty, resource therefore idle. + * + * It is possible that a resource has not been used for a long time and is idle, but the head and + * tail wrapped around in such a way that the previously set access time falls between head and tail. + * In this case we will incorrectly wait for the resource. Because we use the entire 32 bits of the + * counters and not just the bits needed to address the actual queue memory, this should happen rarely. + * If it turns out to be a problem we can switch to 64 bit counters or attempt to somehow mark the + * access time of resources invalid. CS packets are at least 4 byte aligned, so we could use the lower + * 2 bits in access_time for such a marker. + * + * Note that the access time is set before the command is submitted, so we have to wait until the + * tail is bigger than access_time, not equal. */ + access_time = resource->access_time; + head = cs->queue[WINED3D_CS_QUEUE_DEFAULT].head; + while (1) + { + tail = *(volatile ULONG *)&cs->queue[WINED3D_CS_QUEUE_DEFAULT].tail; + if (head == tail) /* Case 6, queue empty. */ + break; + + if (head > tail) + { + if (access_time >= head || access_time < tail) /* Case 2, 3. */ + break; + } + else if (access_time > tail || access_time <= head) /* Case 5. */ + { + break; + } + /* Case 1, 4 - busy, wait a little. */ YieldProcessor(); + } }
- Jan