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-rw-r--r--nptl/pthread_cancel.c6
1 files changed, 6 insertions, 0 deletions
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 1523da82aa..8d1b8640f8 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -65,6 +65,12 @@ pthread_cancel (th)
 	     thread as canceled.  */
 	  INTERNAL_SYSCALL_DECL (err);
 
+	  /* One comment: The PID field in the TCB can temporarily be
+	     changed (in fork).  But this must not affect this code
+	     here.  Since this function would have to be called while
+	     the thread is executing fork, it would have to happen in
+	     a signal handler.  But this is no allowed, pthread_cancel
+	     is not guaranteed to be async-safe.  */
 	  int val;
 #if __ASSUME_TGKILL
 	  val = INTERNAL_SYSCALL (tgkill, err, 3,