[PATCH v6 2/5] ntdll: Make server_select a memory barrier.

[Torge Matthies]

From: Torge Matthies

---
 dlls/ntdll/unix/server.c | 4 ++++
 server/thread.c          | 5 +++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/dlls/ntdll/unix/server.c b/dlls/ntdll/unix/server.c
index 51a83f472e1..c52daa18cce 100644
--- a/dlls/ntdll/unix/server.c
+++ b/dlls/ntdll/unix/server.c
@@ -602,6 +602,10 @@ unsigned int server_select( const select_op_t *select_op, data_size_t size, UINT
     sigset_t old_set;
     int signaled;

+    /* Ensure writes so far are visible to other threads, so that we can avoid sending a memory barrier APC
+     * from NtFlushProcessWriteBuffers to this thread while it is sleeping. */
+    MemoryBarrier();
+
     memset( &result, 0, sizeof(result) );

do
diff --git a/server/thread.c b/server/thread.c
index 9eed1e5bbdf..bf3bda0a058 100644
--- a/server/thread.c
+++ b/server/thread.c
@@ -2112,8 +2112,9 @@ DECL_HANDLER(flush_process_write_buffers)
         struct thread_apc *apc;
         int success;

-        /* Do not send a memory barrier APC to the current thread or a terminated thread. */
-        if (thread == current || thread->state == TERMINATED) continue;
+        /* Do not send a memory barrier APC to the current thread or a terminated thread. A memory barrier is also
+         * unnecessary for waiting threads, because server_select performs a memory barrier. */
+        if (thread == current || thread->state == TERMINATED || thread->wait) continue;

if (!(apc = create_apc( &current->obj, &call ))) break;