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Read/write the cross-process diagnostic counters atomically

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present_calls, frames_dropped (VideoShare) and frames_rendered (AudioStreamInfo)
were plain `+= 1` / stores in the DLL, read by the host cross-process. On an x86
DLL a 64-bit store is two halves, so the x64 host could read a torn value during
a carry. Benign (display-only), but a real data race.

Use std::atomic_ref at the access sites rather than changing the field types:
the structs stay plain POD so the layout/offset asserts are unchanged and
AudioStreamInfo stays trivially copyable (it's published/read as a whole struct).
The DLL writers (note_present / note_video_dropped / note_audio_frames) and the
host readers (IpcServer::video_share / hook_status) now use relaxed atomic_ref;
hook_status reloads frames_rendered atomically after the wholesale struct copy.
The dev-tool readers (vk_validate, audio_probe) keep plain reads -- diagnostics of
diagnostics, and same-bitness in practice.

Validated by present_hook_test (present_calls via atomic_ref) and audio_hook_test
(frames_rendered) -- also confirms no atomic_ref alignment fault.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
BlackMark
2026-06-24 01:27:18 +02:00
parent af129f8cfa
commit 66dd003c4c
4 changed files with 31 additions and 13 deletions
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19
host/src/ipc/ipc_server.cpp
View File

@@ -1,8 +1,22 @@
#include "ipc/ipc_server.hpp"
#include <atomic>
#include <cstdint>
namespace coop
{
namespace
{
// The hook writes these cumulative diagnostic counters cross-process (an x86 DLL can do a 64-bit
// store in two halves), so read them atomically to avoid a torn value. The shared mapping is
// genuinely mutable -- the const here is just our read-only view -- so const_cast for atomic_ref.
std::uint64_t atomic_load_u64(const std::uint64_t& field)
{
return std::atomic_ref(const_cast<std::uint64_t&>(field)).load(std::memory_order_relaxed);
}
} // namespace
bool IpcServer::start(unsigned long target_pid)
{
std::scoped_lock lock(mutex_);
@@ -83,6 +97,7 @@ HookStatusView IpcServer::hook_status() const
for (std::uint32_t i = 0; i < kMaxAudioStreams; ++i)
{
view.audio_streams[i] = s.audio_streams[i];
view.audio_streams[i].frames_rendered = atomic_load_u64(s.audio_streams[i].frames_rendered);
}
view.hook_entry_count = s.hook_entry_count;
for (std::uint32_t i = 0; i < kMaxHookEntries; ++i)
@@ -111,9 +126,9 @@ VideoShareView IpcServer::video_share() const
v.width = s.width;
v.height = s.height;
v.format = s.format;
v.present_calls = s.present_calls;
v.present_calls = atomic_load_u64(s.present_calls);
v.present_qpc = s.present_qpc;
v.frames_dropped = s.frames_dropped;
v.frames_dropped = atomic_load_u64(s.frames_dropped);
return v;
}