Changeset 4db6eaf in mainline for uspace/lib/libc/generic/futex.c

Timestamp:

2008-01-06T14:05:15Z (17 years ago)

Author:

Jakub Jermar <jakub@…>

Branches:

lfn, master, serial, ticket/834-toolchain-update, topic/msim-upgrade, topic/simplify-dev-export

Children:

10d6b858

Parents:

b3c38750

Message:

Fix type in declaration of VFS node rwlock.
Introduce a dedicated type for futexes. Make the existing futex code use this
newly introduced type and fix formatting in futex.c.

File:

• uspace/lib/libc/generic/futex.c (modified) (6 diffs)

uspace/lib/libc/generic/futex.c

@@ -1 +1 @@
 /*
- * Copyright (c) 2006 Jakub Jermar
+ * Copyright (c) 2008 Jakub Jermar
  * All rights reserved.
  *
@@ -42 +42 @@
 /*
  * Note about race conditions.
- * Because of non-atomic nature of operations performed sequentially on the futex
- * counter and the futex wait queue, there is a race condition:
+ * Because of non-atomic nature of operations performed sequentially on the
+ * futex counter and the futex wait queue, there is a race condition:
  *
  * (wq->missed_wakeups == 1) && (futex->count = 1)
@@ -55 +55 @@
  * 5. B has to call SYS_FUTEX_WAKEUP syscall to wake up the sleeping thread
  * 6. B finds the wait queue empty and changes wq->missed_wakeups from 0 to 1
- * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
+ * 7. A fixes futex->count (i.e. the number of waiting threads) by changing it
+ *    from 0 to 1
  *
  * Scenario 2 (conditional down operation vs. futex_up)
  * 1. assume wq->missed_wakeups == 0 && futex->count == 0
  *    (i.e. thread A is in the critical section)
- * 2. thread B performs futex_trydown() operation and changes futex->count from 0 to -1
+ * 2. thread B performs futex_trydown() operation and changes futex->count from
+ *    0 to -1
  *    B is now obliged to call SYS_FUTEX_SLEEP syscall
  * 3. A wants to leave the critical section and does futex_up()
- * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP syscall
+ * 4. A thus changes futex->count from -1 to 0 and must call SYS_FUTEX_WAKEUP
+ *    syscall
  * 5. B finds the wait queue empty and immediatelly aborts the conditional sleep
- * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from 0 to 1
- * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it from 0 to 1
+ * 6. No thread is queueing in the wait queue so wq->missed_wakeups changes from
+ *    0 to 1
+ * 6. B fixes futex->count (i.e. the number of waiting threads) by changing it
+ *    from 0 to 1
  *
- * Both scenarios allow two threads to be in the critical section simultaneously.
- * One without kernel intervention and the other through wq->missed_wakeups being 1.
+ * Both scenarios allow two threads to be in the critical section
+ * simultaneously. One without kernel intervention and the other through
+ * wq->missed_wakeups being 1.
  *
- * To mitigate this problem, futex_down_timeout() detects that the syscall didn't sleep
- * in the wait queue, fixes the futex counter and RETRIES the whole operation again.
- *
+ * To mitigate this problem, futex_down_timeout() detects that the syscall
+ * didn't sleep in the wait queue, fixes the futex counter and RETRIES the
+ * whole operation again.
  */
 
 /** Initialize futex counter.
  *
- * @param futex Futex.
- * @param val Initialization value.
+ * @param futex         Futex.
+ * @param val           Initialization value.
  */
-void futex_initialize(atomic_t *futex, int val)
+void futex_initialize(futex_t *futex, int val)
 {
         atomic_set(futex, val);
 }
 
-int futex_down(atomic_t *futex)
+int futex_down(futex_t *futex)
 {
         return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NONE);
 }
 
-int futex_trydown(atomic_t *futex)
+int futex_trydown(futex_t *futex)
 {
-        return futex_down_timeout(futex, SYNCH_NO_TIMEOUT, SYNCH_FLAGS_NON_BLOCKING);
+        return futex_down_timeout(futex, SYNCH_NO_TIMEOUT,
+            SYNCH_FLAGS_NON_BLOCKING);
 }
 
 /** Try to down the futex.
  *
- * @param futex Futex.
- * @param usec Microseconds to wait. Zero value means sleep without timeout.
- * @param flags Select mode of operation. See comment for waitq_sleep_timeout(). 
+ * @param futex         Futex.
+ * @param usec          Microseconds to wait. Zero value means sleep without
+ *                      timeout.
+ * @param flags         Select mode of operation. See comment for
+ *                      waitq_sleep_timeout(). 
  *
- * @return ENOENT if there is no such virtual address. One of ESYNCH_OK_ATOMIC
- *         and ESYNCH_OK_BLOCKED on success or ESYNCH_TIMEOUT if the lock was
- *         not acquired because of a timeout or ESYNCH_WOULD_BLOCK if the
- *         operation could not be carried out atomically (if requested so).
+ * @return              ENOENT if there is no such virtual address. One of
+ *                      ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED on success or
+ *                      ESYNCH_TIMEOUT if the lock was not acquired because of
+ *                      a timeout or ESYNCH_WOULD_BLOCK if the operation could
+ *                      not be carried out atomically (if requested so).
  */
-int futex_down_timeout(atomic_t *futex, uint32_t usec, int flags)
+int futex_down_timeout(futex_t *futex, uint32_t usec, int flags)
 {
         int rc;
         
         while (atomic_predec(futex) < 0) {
-                rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count, (sysarg_t) usec, (sysarg_t) flags);
+                rc = __SYSCALL3(SYS_FUTEX_SLEEP, (sysarg_t) &futex->count,
+                    (sysarg_t) usec, (sysarg_t) flags);
                 
                 switch (rc) {
                 case ESYNCH_OK_ATOMIC:
                         /*
-                         * Because of a race condition between timeout and futex_up()
-                         * and between conditional futex_down_timeout() and futex_up(),
-                         * we have to give up and try again in this special case.
+                         * Because of a race condition between timeout and
+                         * futex_up() and between conditional
+                         * futex_down_timeout() and futex_up(), we have to give
+                         * up and try again in this special case.
                          */
                         atomic_inc(futex);
@@ -131 +143 @@
                 case ESYNCH_WOULD_BLOCK:
                         /*
-                         * The conditional down operation should be implemented this way.
-                         * The userspace-only variant tends to accumulate missed wakeups
-                         * in the kernel futex wait queue.
+                         * The conditional down operation should be implemented
+                         * this way. The userspace-only variant tends to
+                         * accumulate missed wakeups in the kernel futex wait
+                         * queue.
                          */
                         atomic_inc(futex);
@@ -142 +155 @@
                         /*
                          * Enter the critical section.
-                         * The futex counter has already been incremented for us.
+                         * The futex counter has already been incremented for
+                         * us.
                          */
                         return ESYNCH_OK_BLOCKED;
@@ -159 +173 @@
 /** Up the futex.
  *
- * @param futex Futex.
+ * @param futex         Futex.
  *
- * @return ENOENT if there is no such virtual address. Otherwise zero.
+ * @return              ENOENT if there is no such virtual address. Otherwise
+ *                      zero.
  */
-int futex_up(atomic_t *futex)
+int futex_up(futex_t *futex)
 {
         long val;