Changeset 508b0df1 in mainline for uspace/lib/c/generic/private

Timestamp:

2018-09-06T20:21:52Z (7 years ago)

Author:

Jiří Zárevúcky <jiri.zarevucky@…>

Branches:

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

Children:

78de83de, fc10e1b

Parents:

4621d23

git-author:

Jiří Zárevúcky <jiri.zarevucky@…> (2018-08-13 03:53:39)

git-committer:

Jiří Zárevúcky <jiri.zarevucky@…> (2018-09-06 20:21:52)

Message:

Remove uspace <atomic.h>, use <stdatomic.h> instead

File:

• uspace/lib/c/generic/private/futex.h (modified) (9 diffs)

uspace/lib/c/generic/private/futex.h

@@ -37 +37 @@
 
 #include <assert.h>
-#include <atomic.h>
+#include <stdatomic.h>
 #include <errno.h>
 #include <libc.h>
@@ -43 +43 @@
 
 typedef struct futex {
-        atomic_t val;
+        volatile atomic_int val;
 #ifdef CONFIG_DEBUG_FUTEX
         void *owner;
@@ -53 +53 @@
 #ifdef CONFIG_DEBUG_FUTEX
 
-#define FUTEX_INITIALIZE(val) {{ (val) }, NULL }
+#define FUTEX_INITIALIZE(val) { (val) , NULL }
 #define FUTEX_INITIALIZER     FUTEX_INITIALIZE(1)
 
@@ -73 +73 @@
 #else
 
-#define FUTEX_INITIALIZE(val) {{ (val) }}
+#define FUTEX_INITIALIZE(val) { (val) }
 #define FUTEX_INITIALIZER     FUTEX_INITIALIZE(1)
 
@@ -107 +107 @@
         // TODO: Add tests for this.
 
-        if ((atomic_signed_t) atomic_predec(&futex->val) >= 0)
+        if (atomic_fetch_sub_explicit(&futex->val, 1, memory_order_acquire) > 0)
                 return EOK;
+
+        /* There wasn't any token. We must defer to the underlying semaphore. */
 
         usec_t timeout;
@@ -129 +131 @@
         }
 
-        return __SYSCALL2(SYS_FUTEX_SLEEP, (sysarg_t) &futex->val.count, (sysarg_t) timeout);
+        return __SYSCALL2(SYS_FUTEX_SLEEP, (sysarg_t) futex, (sysarg_t) timeout);
 }
 
@@ -143 +145 @@
 static inline errno_t futex_up(futex_t *futex)
 {
-        if ((atomic_signed_t) atomic_postinc(&futex->val) < 0)
-                return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) &futex->val.count);
+        if (atomic_fetch_add_explicit(&futex->val, 1, memory_order_release) < 0)
+                return __SYSCALL1(SYS_FUTEX_WAKEUP, (sysarg_t) futex);
 
         return EOK;
@@ -152 +154 @@
     const struct timespec *expires)
 {
-        if (expires && expires->tv_sec == 0 && expires->tv_nsec == 0) {
-                /* Nonblocking down. */
-
-                /*
-                 * Try good old CAS a few times.
-                 * Not too much though, we don't want to bloat the caller.
-                 */
-                for (int i = 0; i < 2; i++) {
-                        atomic_signed_t old = atomic_get(&futex->val);
-                        if (old <= 0)
-                                return ETIMEOUT;
-
-                        if (cas(&futex->val, old, old - 1))
-                                return EOK;
-                }
-
-                // TODO: builtin atomics with relaxed ordering can make this
-                //       faster.
-
-                /*
-                 * If we don't succeed with CAS, we can't just return failure
-                 * because that would lead to spurious failures where
-                 * futex_down_timeout returns ETIMEOUT despite there being
-                 * available tokens. That could break some algorithms.
-                 * We also don't want to loop on CAS indefinitely, because
-                 * that would make the semaphore not wait-free, even when all
-                 * atomic operations and the underlying base semaphore are all
-                 * wait-free.
-                 * Instead, we fall back to regular down_timeout(), with
-                 * an already expired deadline. That way we delegate all these
-                 * concerns to the base semaphore.
-                 */
-        }
-
         /*
          * This combination of a "composable" sleep followed by futex_up() on
@@ -208 +176 @@
 {
         /*
-         * down_timeout with an already expired deadline should behave like
-         * trydown.
+         * We can't just use CAS here.
+         * If we don't succeed with CAS, we can't return failure
+         * because that would lead to spurious failures where
+         * futex_down_timeout returns ETIMEOUT despite there being
+         * available tokens. That would break some algorithms.
+         * We also don't want to loop on CAS indefinitely, because
+         * that would make the semaphore not wait-free, even when all
+         * atomic operations and the underlying base semaphore are all
+         * wait-free.
+         * It's much less trouble (and code bloat) to just do regular
+         * down_timeout(), with an already expired deadline.
          */
         struct timespec tv = { .tv_sec = 0, .tv_nsec = 0 };