Changeset b59318e in mainline for uspace/lib/c/include/futex.h

Timestamp:

2018-06-26T17:34:48Z (6 years ago)

Author:

Jiří Zárevúcky <jiri.zarevucky@…>

Branches:

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

Children:

ab6edb6

Parents:

f6372be9

git-author:

Jiří Zárevúcky <jiri.zarevucky@…> (2018-06-26 17:01:43)

git-committer:

Jiří Zárevúcky <jiri.zarevucky@…> (2018-06-26 17:34:48)

Message:

Make futex able to time out.

File:

• uspace/lib/c/include/futex.h (modified) (3 diffs)

uspace/lib/c/include/futex.h

@@ -36 +36 @@
 #define LIBC_FUTEX_H_
 
+#include <assert.h>
 #include <atomic.h>
 #include <errno.h>
@@ -98 +99 @@
 }
 
-/** Down the futex.
+/** Down the futex with timeout, composably.
+ *
+ * This means that when the operation fails due to a timeout or being
+ * interrupted, the next futex_up() is ignored, which allows certain kinds of
+ * composition of synchronization primitives.
+ *
+ * In most other circumstances, regular futex_down_timeout() is a better choice.
  *
  * @param futex Futex.
  *
  * @return ENOENT if there is no such virtual address.
+ * @return ETIMEOUT if timeout expires.
  * @return EOK on success.
  * @return Error code from <errno.h> otherwise.
  *
  */
-static inline errno_t futex_down(futex_t *futex)
+static inline errno_t futex_down_composable(futex_t *futex, struct timeval *expires)
 {
+        // TODO: Add tests for this.
+
+        /* No timeout by default. */
+        suseconds_t timeout = 0;
+
+        if (expires) {
+                struct timeval tv;
+                getuptime(&tv);
+                if (tv_gteq(&tv, expires)) {
+                        /* We can't just return ETIMEOUT. That wouldn't be composable. */
+                        timeout = 1;
+                } else {
+                        timeout = tv_sub_diff(expires, &tv);
+                }
+
+                assert(timeout > 0);
+        }
+
         if ((atomic_signed_t) atomic_predec(&futex->val) < 0)
-                return (errno_t) __SYSCALL1(SYS_FUTEX_SLEEP, (sysarg_t) &futex->val.count);
+                return (errno_t) __SYSCALL2(SYS_FUTEX_SLEEP, (sysarg_t) &futex->val.count, (sysarg_t) timeout);
 
         return EOK;
@@ -132 +158 @@
 }
 
+static inline errno_t futex_down_timeout(futex_t *futex, struct timeval *expires)
+{
+        /*
+         * This combination of a "composable" sleep followed by futex_up() on
+         * failure is necessary to prevent breakage due to certain race
+         * conditions.
+         */
+        errno_t rc = futex_down_composable(futex, expires);
+        if (rc != EOK)
+                futex_up(futex);
+        return rc;
+}
+
+/** Down the futex.
+ *
+ * @param futex Futex.
+ *
+ * @return ENOENT if there is no such virtual address.
+ * @return EOK on success.
+ * @return Error code from <errno.h> otherwise.
+ *
+ */
+static inline errno_t futex_down(futex_t *futex)
+{
+        return futex_down_timeout(futex, NULL);
+}
+
 #endif