rom/exec/obtainsemaphorelist.c

   1 /*
   2     Copyright © 1995-2015, The AROS Development Team. All rights reserved.
   3     $Id$
   4
   5     Desc: Lock all semaphores in the list at once.
   6     Lang: english
   7 */
   8 #include "exec_intern.h"
   9 #include "semaphores.h"
  10 #include <exec/semaphores.h>
  11 #include <proto/exec.h>
  12
  13 /*****************************************************************************
  14
  15     NAME */
  16
  17         AROS_LH1(void, ObtainSemaphoreList,
  18
  19 /*  SYNOPSIS */
  20         AROS_LHA(struct List *, sigSem, A0),
  21
  22 /*  LOCATION */
  23         struct ExecBase *, SysBase, 97, Exec)
  24
  25 /*  FUNCTION
  26         This function takes a list of semaphores and locks all of them at
  27         once. It is only possible for one task to attempt to lock all the
  28         semaphores at once (since it uses the ss_MultipleLink field), so
  29         you will need to protect the entire list (with another semaphore
  30         perhaps?).
  31
  32         If somebody attempts to lock more than one semaphore on this list
  33         with ObtainSemaphore() it is possible for a deadlock to occur due
  34         to two tasks waiting for a semaphore that the other has obtained.
  35
  36     INPUTS
  37         sigSem - pointer to list full of semaphores
  38
  39     RESULT
  40         The entire semaphore list will be locked.
  41
  42     NOTES
  43
  44     EXAMPLE
  45
  46     BUGS
  47
  48     SEE ALSO
  49
  50     INTERNALS
  51
  52 *****************************************************************************/
  53 {
  54     AROS_LIBFUNC_INIT
  55
  56     struct SignalSemaphore *ss;
  57     struct Task * const ThisTask = GET_THIS_TASK;
  58     WORD failedObtain = 0;
  59
  60     /*
  61      *  The algorithm here is to attempt to lock all the semaphores in the
  62      *  list, and if any fail, to post a SemaphoreRequest for the
  63      *  semaphore.
  64      *
  65      *  If we succeed in locking them all, we can return successfully,
  66      *  otherwise we must wait for the remaining semaphores to become
  67      *  available.
  68      *
  69      *  Note that we sleep on each un-obtained semaphore as we pass through
  70      *  the list. This way by the time we get to the end of the list we can
  71      *  be sure that we have obtained all the semaphores. It is possible
  72      *  that whilst we are waiting for one semaphore, one later in the list
  73      *  will be granted to us. In that case we do not have to wait for it.
  74      */
  75
  76     Forbid();
  77
  78     ForeachNode(sigSem, ss)
  79     {
  80         /* QueueCount == -1 means unlocked */
  81         ss->ss_QueueCount++;
  82         if(ss->ss_QueueCount != 0)
  83         {
  84             /* sem already locked by ThisTask? */
  85             if (ss->ss_Owner != ThisTask)
  86             {
  87                 /*
  88                  *      Locked by someone else, post a wait message. We use the field
  89                  *      ss_MultipleLink, which is why this function requires an
  90                  *      external arbitrator.
  91                  */
  92                 ss->ss_MultipleLink.sr_Waiter = ThisTask;
  93                 AddTail
  94                 (
  95                     (struct List *)&ss->ss_WaitQueue,
  96                     (struct Node *)&ss->ss_MultipleLink
  97                 );
  98                 failedObtain++;
  99             }
 100             else
 101             {
 102                 /* Already locked by ThisTask */
 103                 ss->ss_NestCount++;
 104             }
 105         }
 106         else
 107         {
 108             /* We have it... */
 109             ss->ss_NestCount++;
 110             ss->ss_Owner = ThisTask;
 111         }
 112     }
 113
 114     if(failedObtain > 0)
 115     {
 116         ss = (struct SignalSemaphore *)sigSem->lh_Head;
 117
 118         while(ss->ss_Link.ln_Succ != NULL)
 119         {
 120             if(ss->ss_Owner != ThisTask)
 121             {
 122                 /*
 123                  *  Somebody else has this one. Wait, then check again.
 124                  *  Check again because the signal could have been for a
 125                  *  different semaphore in the list we are waiting for.
 126                  */
 127                 Wait(SIGF_SINGLE);
 128             }
 129             else
 130             {
 131                 /* We got it, go on to the next one */
 132                 ss = (struct SignalSemaphore *)ss->ss_Link.ln_Succ;
 133                 failedObtain--;
 134             }
 135         }
 136     }
 137
 138 #ifndef NO_CONSISTENCY_CHECKS
 139     if(failedObtain != 0)
 140     {
 141         kprintf("\n\nObtainSemaList: Obtained count mismatch %d\n", failedObtain);
 142         Alert(AN_BadSemaphore);
 143     }
 144 #endif
 145
 146     Permit();
 147
 148     AROS_LIBFUNC_EXIT
 149 } /* ObtainSemaphoreList */