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Re: [Qemu-devel] [PATCH] thread-win32: fix GetThreadContext() permanentl


From: Stefan Weil
Subject: Re: [Qemu-devel] [PATCH] thread-win32: fix GetThreadContext() permanently fails
Date: Tue, 23 Jun 2015 08:02:49 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.7.0

Am 22.06.2015 um 23:54 schrieb Zavadovsky Yan:
Calling SuspendThread() is not enough to suspend Win32 thread.
We need to call GetThreadContext() after SuspendThread()
to make sure that OS have really suspended target thread.
But GetThreadContext() needs for THREAD_GET_CONTEXT
access right on thread object.

This patch adds THREAD_GET_CONTEXT to OpenThread() arguments
and change 'while(GetThreadContext() == SUCCESS)' to
'while(GetThreadContext() == FAILED)'.
So this 'while' loop will stop only after successful grabbing
of thread context(i.e. when thread is really suspended).
Not after the one failed GetThreadContext() call.

Signed-off-by: Zavadovsky Yan <address@hidden>
---
  cpus.c                   | 2 +-
  util/qemu-thread-win32.c | 4 ++--
  2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/cpus.c b/cpus.c
index b85fb5f..83d5eb5 100644
--- a/cpus.c
+++ b/cpus.c
@@ -1097,7 +1097,7 @@ static void qemu_cpu_kick_thread(CPUState *cpu)
           * suspended until we can get the context.
           */
          tcgContext.ContextFlags = CONTEXT_CONTROL;
-        while (GetThreadContext(cpu->hThread, &tcgContext) != 0) {
+        while (GetThreadContext(cpu->hThread, &tcgContext) == 0) {
              continue;
          }
diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index 406b52f..823eca1 100644
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -406,8 +406,8 @@ HANDLE qemu_thread_get_handle(QemuThread *thread)
EnterCriticalSection(&data->cs);
      if (!data->exited) {
-        handle = OpenThread(SYNCHRONIZE | THREAD_SUSPEND_RESUME, FALSE,
-                            thread->tid);
+        handle = OpenThread(SYNCHRONIZE | THREAD_SUSPEND_RESUME | 
THREAD_GET_CONTEXT,
+                            FALSE, thread->tid);
      } else {
          handle = NULL;
      }


I added the contributers of the original code to the cc list.

The modifications look reasonable - if GetThreadContext is needed at all.
We should add an URL to reliable documentation which supports that
claim.

Is it a good idea to run a busy waiting loop? Or would a Sleep(0) in
the loop be better (it allows other threads to run, maybe it helps
them to suspend, too).

Regards
Stefan




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