Merge pull request #5596 from kjbracey-arm/strong_cas

Make LDREX/STREX CAS functions strong

Author: 0xc0170

platform/mbed_critical.c

@@ -110,39 +110,42 @@ MBED_WEAK void core_util_critical_section_exit(void)
 
 bool core_util_atomic_cas_u8(uint8_t *ptr, uint8_t *expectedCurrentValue, uint8_t desiredValue)
 {
-    uint8_t currentValue = __LDREXB((volatile uint8_t*)ptr);
-    if (currentValue != *expectedCurrentValue) {
-        *expectedCurrentValue = currentValue;
-        __CLREX();
-        return false;
-    }
-
-    return !__STREXB(desiredValue, (volatile uint8_t*)ptr);
+    do {
+        uint8_t currentValue = __LDREXB((volatile uint8_t*)ptr);
+        if (currentValue != *expectedCurrentValue) {
+            *expectedCurrentValue = currentValue;
+            __CLREX();
+            return false;
+        }
+    } while (__STREXB(desiredValue, (volatile uint8_t*)ptr));
+    return true;
 }
 
 bool core_util_atomic_cas_u16(uint16_t *ptr, uint16_t *expectedCurrentValue, uint16_t desiredValue)
 {
-    uint16_t currentValue = __LDREXH((volatile uint16_t*)ptr);
-    if (currentValue != *expectedCurrentValue) {
-        *expectedCurrentValue = currentValue;
-        __CLREX();
-        return false;
-    }
-
-    return !__STREXH(desiredValue, (volatile uint16_t*)ptr);
+    do {
+        uint16_t currentValue = __LDREXH((volatile uint16_t*)ptr);
+        if (currentValue != *expectedCurrentValue) {
+            *expectedCurrentValue = currentValue;
+            __CLREX();
+            return false;
+        }
+    } while (__STREXH(desiredValue, (volatile uint16_t*)ptr));
+    return true;
 }
 
 
 bool core_util_atomic_cas_u32(uint32_t *ptr, uint32_t *expectedCurrentValue, uint32_t desiredValue)
 {
-    uint32_t currentValue = __LDREXW((volatile uint32_t*)ptr);
-    if (currentValue != *expectedCurrentValue) {
-        *expectedCurrentValue = currentValue;
-        __CLREX();
-        return false;
-    }
-
-    return !__STREXW(desiredValue, (volatile uint32_t*)ptr);
+    do {
+        uint32_t currentValue = __LDREXW((volatile uint32_t*)ptr);
+        if (currentValue != *expectedCurrentValue) {
+            *expectedCurrentValue = currentValue;
+            __CLREX();
+            return false;
+        }
+    } while (__STREXW(desiredValue, (volatile uint32_t*)ptr));
+    return true;
 }
 
 uint8_t core_util_atomic_incr_u8(uint8_t *valuePtr, uint8_t delta)

platform/mbed_critical.h

@@ -121,7 +121,7 @@ void core_util_critical_section_exit(void);
  * }
  *
  * @note: In the failure case (where the destination isn't set), the value
- * pointed to by expectedCurrentValue is still updated with the current value.
+ * pointed to by expectedCurrentValue is instead updated with the current value.
  * This property helps writing concise code for the following incr:
  *
  * function incr(p : pointer to int, a : int) returns int {
@@ -132,6 +132,10 @@ void core_util_critical_section_exit(void);
  *     }
  *     return value + a
  * }
+ *
+ * @note: This corresponds to the C11 "atomic_compare_exchange_strong" - it
+ * always succeeds if the current value is expected, as per the pseudocode
+ * above; it will not spuriously fail as "atomic_compare_exchange_weak" may.
  */
 bool core_util_atomic_cas_u8(uint8_t *ptr, uint8_t *expectedCurrentValue, uint8_t desiredValue);
 
@@ -174,7 +178,7 @@ bool core_util_atomic_cas_u8(uint8_t *ptr, uint8_t *expectedCurrentValue, uint8_
  * }
  *
  * @note: In the failure case (where the destination isn't set), the value
- * pointed to by expectedCurrentValue is still updated with the current value.
+ * pointed to by expectedCurrentValue is instead updated with the current value.
  * This property helps writing concise code for the following incr:
  *
  * function incr(p : pointer to int, a : int) returns int {
@@ -185,6 +189,10 @@ bool core_util_atomic_cas_u8(uint8_t *ptr, uint8_t *expectedCurrentValue, uint8_
  *     }
  *     return value + a
  * }
+ *
+ * @note: This corresponds to the C11 "atomic_compare_exchange_strong" - it
+ * always succeeds if the current value is expected, as per the pseudocode
+ * above; it will not spuriously fail as "atomic_compare_exchange_weak" may.
  */
 bool core_util_atomic_cas_u16(uint16_t *ptr, uint16_t *expectedCurrentValue, uint16_t desiredValue);
 
@@ -227,7 +235,7 @@ bool core_util_atomic_cas_u16(uint16_t *ptr, uint16_t *expectedCurrentValue, uin
  * }
  *
  * @note: In the failure case (where the destination isn't set), the value
- * pointed to by expectedCurrentValue is still updated with the current value.
+ * pointed to by expectedCurrentValue is instead updated with the current value.
  * This property helps writing concise code for the following incr:
  *
  * function incr(p : pointer to int, a : int) returns int {
@@ -237,6 +245,10 @@ bool core_util_atomic_cas_u16(uint16_t *ptr, uint16_t *expectedCurrentValue, uin
  *         done = atomic_cas(p, &value, value + a) // *value gets updated automatically until success
  *     }
  *     return value + a
+ *
+ * @note: This corresponds to the C11 "atomic_compare_exchange_strong" - it
+ * always succeeds if the current value is expected, as per the pseudocode
+ * above; it will not spuriously fail as "atomic_compare_exchange_weak" may.
  * }
  */
 bool core_util_atomic_cas_u32(uint32_t *ptr, uint32_t *expectedCurrentValue, uint32_t desiredValue);
@@ -280,7 +292,7 @@ bool core_util_atomic_cas_u32(uint32_t *ptr, uint32_t *expectedCurrentValue, uin
  * }
  *
  * @note: In the failure case (where the destination isn't set), the value
- * pointed to by expectedCurrentValue is still updated with the current value.
+ * pointed to by expectedCurrentValue is instead updated with the current value.
  * This property helps writing concise code for the following incr:
  *
  * function incr(p : pointer to int, a : int) returns int {
@@ -291,6 +303,10 @@ bool core_util_atomic_cas_u32(uint32_t *ptr, uint32_t *expectedCurrentValue, uin
  *     }
  *     return value + a
  * }
+ *
+ * @note: This corresponds to the C11 "atomic_compare_exchange_strong" - it
+ * always succeeds if the current value is expected, as per the pseudocode
+ * above; it will not spuriously fail as "atomic_compare_exchange_weak" may.
  */
 bool core_util_atomic_cas_ptr(void **ptr, void **expectedCurrentValue, void *desiredValue);