There are common patterns in the kernel of using test_and_clear_bit()
before wake_up_bit(), and atomic_dec_and_test() before wake_up_var().
These combinations don't need extra barriers but sometimes include them
unnecessarily.
To help avoid the unnecessary barriers and to help discourage the
general use of wake_up_bit/var (which is a fragile interface) introduce
two combined functions which implement these patterns.
Also add store_release_wake_up() which support the parts of simply
setting a non-atomic variable as sending a wakeup. This pattern
requires barriers which are often omitted.
Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
include/linux/wait_bit.h | 61 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 61 insertions(+)
diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h
index ca5c6e70f908..c1675457c077 100644
--- a/include/linux/wait_bit.h
+++ b/include/linux/wait_bit.h
@@ -419,4 +419,65 @@ static inline void clear_and_wake_up_bit(int bit, unsigned long *word)
wake_up_bit(word, bit);
}
+/**
+ * test_and_clear_wake_up_bit - clear a bit if it was set: wake up anyone waiting on that bit
+ *
+ * @bit: the bit of the word being waited on
+ * @word: the address of memory containing that bit
+ *
+ * If the bit is set and can be atomically cleared, any tasks waiting in
+ * wait_on_bit() or similar will be woken. This call has the same
+ * complete ordering semantics as test_and_clear_bit(). Any changes to
+ * memory made before this call are guaranteed to be visible after the
+ * corresponding wait_on_bit() completes.
+ *
+ * Returns %true if the bit was successfully set and the wake up was sent.
+ */
+static inline bool test_and_clear_wake_up_bit(int bit, unsigned long *word)
+{
+ if (!test_and_clear_bit(bit, word))
+ return false;
+ /* no extra barrier required */
+ wake_up_bit(word, bit);
+ return true;
+}
+
+/**
+ * atomic_dec_and_wake_up - decrement an atomic_t and if zero, wake up waiters
+ *
+ * @var: the variable to dec and test
+ *
+ * Decrements the atomic variable and if it reaches zero, send a wake_up to any
+ * processes waiting on the variable.
+ *
+ * This function has the same complete ordering semantics as atomic_dec_and_test.
+ *
+ * Returns %true is the variable reaches zero and the wake up was sent.
+ */
+
+static inline bool atomic_dec_and_wake_up(atomic_t *var)
+{
+ if (!atomic_dec_and_test(var))
+ return false;
+ wake_up_var(var);
+ return true;
+}
+
+/**
+ * store_release_wake_up - update a variable and send a wake_up
+ * @var: the address of the variable to be updated and woken
+ * @val: the value to store in the variable.
+ *
+ * Store the given value in the variable send a wake up to any tasks
+ * waiting on the variable. All necessary barriers are included to ensure
+ * the task calling wait_var_event() sees the new value and all values
+ * written to memory before this call.
+ */
+#define store_release_wake_up(var, val) \
+do { \
+ smp_store_release(var, val); \
+ smp_mb(); \
+ wake_up_var(var); \
+} while (0)
+
#endif /* _LINUX_WAIT_BIT_H */
--
2.44.0
