winsup/cygwin/local_includes/spinlock.h

   1 /* spinlock.h: Header file for cygwin time-sensitive synchronization primitive.
   2
   3 This file is part of Cygwin.
   4
   5 This software is a copyrighted work licensed under the terms of the
   6 Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
   7 details. */
   8
   9 #ifndef _SPINLOCK_H
  10 #define _SPINLOCK_H
  11
  12 #include "ntdll.h"
  13
  14 #define SPINLOCK_WAIT (15000LL * 10000LL)
  15
  16 class spinlock
  17 {
  18   LONG *locker;
  19   LONG val;
  20   LONG setto;
  21   void done (LONG what)
  22   {
  23     if (locker)
  24       {
  25         InterlockedExchange (locker, what);
  26         locker = NULL;
  27       }
  28   }
  29   long long time ()
  30   {
  31     LARGE_INTEGER t;
  32     if (NtQuerySystemTime (&t) == STATUS_SUCCESS)
  33       return t.QuadPart;
  34     return 0LL;
  35   }
  36 public:
  37   spinlock (LONG& locktest, LONG wanted_val = 1, LONGLONG timeout = SPINLOCK_WAIT):
  38     locker (&locktest), setto (wanted_val)
  39   {
  40     /* Quick test to see if we're already initialized */
  41     if ((val = locktest) == wanted_val)
  42       locker = NULL;
  43     /* Slightly less quick test to see if we are the first cygwin process */
  44     else if ((val = InterlockedExchange (locker, -1)) == 0)
  45       /* We're armed and dangerous */;
  46     else if (val == wanted_val)
  47       done (val);       /* This was initialized while we weren't looking */
  48     else
  49       {
  50         long long then = time ();
  51         /* Loop waiting for some other process to set locktest to something
  52            other than -1, indicating that initialization has finished.  Or,
  53            wait a default of 15 seconds for that to happen and, if it doesn't
  54            just grab the lock ourselves. */
  55         while ((val = InterlockedExchange (locker, -1)) == -1
  56                && (time () - then) < timeout)
  57           yield ();
  58         /* Reset the lock back to wanted_value under the assumption that is
  59            what caused the above loop to kick out.  */
  60         if (val == -1)
  61           val = 0;      /* Timed out.  We'll initialize things ourselves. */
  62         else
  63           done (val);   /* Put back whatever was there before, assuming that
  64                            it is actually wanted_val. */
  65       }
  66   }
  67   ~spinlock () {done (setto);}
  68   operator ULONG () const {return (ULONG) val;}
  69   /* FIXME: This should be handled in a more general fashion, probably by
  70      establishing a linked list of spinlocks which are freed on process exit. */
  71   void multiple_cygwin_problem (const char *w, unsigned m, unsigned v)
  72   {
  73     done (val);
  74     ::multiple_cygwin_problem (w, m, v);
  75   }
  76 };
  77
  78 #endif /*_SPINLOCK_H*/