import winsup-2000-02-17 snapshot

1 files changed, 112 insertions, 0 deletions

diff --git a/winsup/cygwin/sync.cc b/winsup/cygwin/sync.cc
new file mode 100644
index 000000000..89d92ce06
--- /dev/null
+++ b/winsup/cygwin/sync.cc
@@ -0,0 +1,112 @@
+/* sync.cc: Synchronization functions for cygwin.
+
+   This file implements the methods for controlling the "muto" class
+   which is intended to operate similarly to a mutex but attempts to
+   avoid making expensive calls to the kernel.
+
+   Copyright 1999 Cygnus Solutions.
+
+   Written by Christopher Faylor <cgf@cygnus.com>
+
+This file is part of Cygwin.
+
+This software is a copyrighted work licensed under the terms of the
+Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
+details. */
+
+#include <stdlib.h>
+#include <time.h>
+#include <sys/wait.h>
+#include <errno.h>
+#include <stdlib.h>
+#include "winsup.h"
+
+/* Constructor */
+muto::muto(int inh, const char *name) : sync (0), visits(0), waiters(-1), tid (0)
+{
+  /* Create event which is used in the fallback case when blocking is necessary */
+  if (!(bruteforce = CreateEvent (inh ? &sec_all_nih : &sec_none_nih, FALSE, FALSE, name)))
+    {
+      DWORD oerr = GetLastError ();
+      SetLastError (oerr);
+      return;
+    }
+}
+
+/* Destructor */
+muto::~muto ()
+{
+  /* Just need to close the event handle */
+  if (bruteforce)
+    CloseHandle (bruteforce);
+}
+
+/* Acquire the lock.  Argument is the number of milliseconds to wait for
+   the lock.  Multiple visits from the same thread are allowed and should
+   be handled correctly.  */
+int
+muto::acquire (DWORD ms)
+{
+  DWORD this_tid = GetCurrentThreadId ();
+
+  if (tid != this_tid)
+    {
+      /* Increment the waiters part of the class.  Need to do this first to
+	 avoid potential races. */
+      LONG was_waiting = InterlockedIncrement (&waiters);
+
+      /* This is deceptively simple.  Basically, it allows multiple attempts to
+	 lock the same muto to succeed without attempting to manipulate sync.
+	 If the muto is already locked then this thread will wait for ms until
+	 it is signalled by muto::release.  Then it will attempt to grab the
+	 sync field.  If it succeeds, then this thread owns the mutex.
+
+	 There is a pathological condition where a thread times out waiting for
+	 bruteforce but the release code triggers the bruteforce event.  In this
+	 case, it is possible for a thread which is going to wait for bruteforce
+	 to wake up immediately.  It will then attempt to grab sync but will fail
+	 and go back to waiting.  */
+      while (tid != this_tid && (was_waiting || InterlockedExchange (&sync, 1) != 0))
+	{
+	  switch (WaitForSingleObject (bruteforce, ms))
+	      {
+	      case WAIT_OBJECT_0:
+		was_waiting = 0;
+		break;
+	      default:
+		InterlockedDecrement (&waiters);
+		return 0;	/* failed. */
+	      }
+	}
+    }
+
+  tid = this_tid;	/* register this thread. */
+  return ++visits;	/* Increment visit count. */
+}
+
+/* Return the muto lock.  Needs to be called once per every acquire. */
+int
+muto::release ()
+{
+  DWORD this_tid = GetCurrentThreadId ();
+
+  if (tid != this_tid || !visits)
+    {
+      SetLastError (ERROR_NOT_OWNER);	/* Didn't have the lock. */
+      return 0;	/* failed. */
+    }
+
+  /* FIXME: Need to check that other thread has not exited, too. */
+  if (!--visits)
+    {
+      tid = 0;		/* We were the last unlocker. */
+      InterlockedExchange (&sync, 0); /* Reset trigger. */
+      /* This thread had incremented waiters but had never decremented it.
+	 Decrement it now.  If it is >= 0 then there are possibly other
+	 threads waiting for the lock, so trigger bruteforce. */
+      if (InterlockedDecrement (&waiters) >= 0)
+	(void) SetEvent (bruteforce); /* Wake up one of the waiting threads */
+    }
+
+  return 1;	/* success. */
+}