[libcamera-devel] [PATCH v4 11/16] py: merge read_event() and get_ready_requests()
Tomi Valkeinen
tomi.valkeinen at ideasonboard.com
Tue May 31 07:51:52 CEST 2022
On 30/05/2022 19:57, Tomi Valkeinen wrote:
>>> diff --git a/src/py/libcamera/py_main.cpp b/src/py/libcamera/py_main.cpp
>>> index fcf009f0..505cc3dc 100644
>>> --- a/src/py/libcamera/py_main.cpp
>>> +++ b/src/py/libcamera/py_main.cpp
>>> @@ -213,15 +213,12 @@ PYBIND11_MODULE(_libcamera, m)
>>> return gEventfd;
>>> })
>>> - .def("read_event", [](CameraManager &) {
>>> + .def("get_ready_requests", [](CameraManager &) {
>>> uint8_t buf[8];
>>> - int ret = read(gEventfd, buf, 8);
>>> - if (ret != 8)
>>> + if (read(gEventfd, buf, 8) != 8)
>>> throw std::system_error(errno,
>>> std::generic_category());
>>
>> We need a memory barrier here to prevent reordering. I'm quite sure
>
> We then need the same in handleRequestCompleted().
>
>> there's one somewhere due to the read() call and the lock below, but I
>> haven't been able to figure out the C++ rule that guarantees this. Does
>> anyone know ?
>
> https://en.cppreference.com/w/cpp/atomic/memory_order
>
> So the mutex brings the normal acquire-release ordering. I would think a
> syscall includes a barrier, but I can't find any details right away.
Perhaps something like this? I don't know if the fences are needed, but maybe
better safe than sorry.
diff --git a/src/py/libcamera/py_main.cpp b/src/py/libcamera/py_main.cpp
index 505cc3dc..b035a101 100644
--- a/src/py/libcamera/py_main.cpp
+++ b/src/py/libcamera/py_main.cpp
@@ -5,6 +5,7 @@
* Python bindings
*/
+#include <atomic>
#include <mutex>
#include <stdexcept>
#include <sys/eventfd.h>
@@ -116,6 +117,12 @@ static void handleRequestCompleted(Request *req)
gReqList.push_back(req);
}
+ /*
+ * Prevent the write below from possibly being reordered to happen
+ * before the push_back() above.
+ */
+ std::atomic_thread_fence(std::memory_order_acquire);
+
uint64_t v = 1;
size_t s = write(gEventfd, &v, 8);
/*
@@ -219,6 +226,12 @@ PYBIND11_MODULE(_libcamera, m)
if (read(gEventfd, buf, 8) != 8)
throw std::system_error(errno, std::generic_category());
+ /*
+ * Prevent the read above from possibly being reordered
+ * to happen after the swap() below.
+ */
+ std::atomic_thread_fence(std::memory_order_release);
+
std::vector<Request *> v;
{
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