Changeset 8a18d5b in mainline

Timestamp:

2018-11-11T15:47:39Z (6 years ago)

Author:

Jakub Jermar <jakub@…>

Branches:

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

Children:

269bc459

Parents:

0b5203b

git-author:

Jakub Jermar <jakub@…> (2018-11-10 16:22:29)

git-committer:

Jakub Jermar <jakub@…> (2018-11-11 15:47:39)

Message:

Preallocate the waitq handle

This fixes a race condition (a missing wakeup) when the wakeup was
faster than the thread going to sleep and no handle was allocated yet.
The handle get preallocated to avoid a possible allocation failure in
wakeup.

We also switched to using atomic_compare_exchange_weak_explicit() to fix
ARMv4 and ARMv5 builds.

Location:

uspace/lib/c/generic

Files:

• private/futex.h (modified) (4 diffs)
• thread/futex.c (modified) (1 diff)

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

@@ -46 +46 @@
 typedef struct futex {
         volatile atomic_int val;
-        volatile atomic_int alloc_lock;
-        cap_waitq_handle_t whandle;
+        volatile atomic_int lock;
+        volatile cap_waitq_handle_t whandle;
 
 #ifdef CONFIG_DEBUG_FUTEX
@@ -91 +91 @@
 #endif
 
+static errno_t allocate_waitq(futex_t *futex)
+{
+        int expected = 0;
+        while (!atomic_compare_exchange_weak_explicit(&futex->lock, &expected,
+            1, memory_order_acquire, memory_order_relaxed))
+                expected = 0;
+
+        if (futex->whandle == CAP_NIL) {
+                errno_t rc = __SYSCALL1(SYS_WAITQ_CREATE,
+                    (sysarg_t) &futex->whandle);
+                if (rc != EOK) {
+                        atomic_store_explicit(&futex->lock, 0,
+                            memory_order_release);
+                        return rc;
+                }
+        }
+
+        atomic_store_explicit(&futex->lock, 0, memory_order_release);
+        return EOK;
+}
+
 /** Down the futex with timeout, composably.
  *
@@ -111 +132 @@
 {
         // TODO: Add tests for this.
+
+        // Preallocate the waitq handle so that we don't need to risk a failure
+        // during wakeup
+        if (futex->whandle == CAP_NIL) {
+                errno_t rc = allocate_waitq(futex);
+                if (rc != EOK)
+                        return rc;
+        }
 
         if (atomic_fetch_sub_explicit(&futex->val, 1, memory_order_acquire) > 0)
@@ -134 +163 @@
 
                 assert(timeout > 0);
-        }
-
-        if (futex->whandle == CAP_NIL) {
-                while (atomic_exchange_explicit(&futex->alloc_lock, 1,
-                    memory_order_acquire) == 1)
-                        ;
-                if (futex->whandle == CAP_NIL) {
-                        errno_t rc = __SYSCALL1(SYS_WAITQ_CREATE,
-                            (sysarg_t) &futex->whandle);
-                        if (rc != EOK) {
-                                atomic_store_explicit(&futex->alloc_lock, 0,
-                                    memory_order_release);
-                                return rc;
-                        }
-                }
-
-                atomic_store_explicit(&futex->alloc_lock, 0,
-                    memory_order_release);
         }
 

uspace/lib/c/generic/thread/futex.c

@@ -53 +53 @@
 {
         atomic_store_explicit(&futex->val, val, memory_order_relaxed);
-        atomic_store_explicit(&futex->alloc_lock, 0, memory_order_relaxed);
+        atomic_store_explicit(&futex->lock, 0, memory_order_relaxed);
         futex->whandle = CAP_NIL;
 }