Re: [PATCH] firmware: wake all waiters

[Davidlohr Bueso]

On Tue, 27 Jun 2017, Luis R. Rodriguez wrote:
>  * As a side effect of this; the data structures are slimmer.
>  *
> - * One would recommend using this wait queue where possible.
> + * NOTE: swait is for cases of extreme memory considerations and some very
> + * special realtime issues, where it saves a couple of bytes in structures that
> + * need close packing. As such its very special-use. Consider using regular
> + * waits queues from wait.h instead *first*.

How about the following?

diff --git a/include/linux/swait.h b/include/linux/swait.h
index 4a4e180d0a35..f72f274f2a5f 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -9,13 +9,16 @@
/*
 * Simple wait queues
 *
- * While these are very similar to the other/complex wait queues (wait.h) the
- * most important difference is that the simple waitqueue allows for
- * deterministic behaviour -- IOW it has strictly bounded IRQ and lock hold
- * times.
+ * While these are very similar to regular wait queues (wait.h) the most
+ * important difference is that the simple waitqueue allows for deterministic
+ * behaviour -- IOW it has strictly bounded IRQ and lock hold times.
 *
- * In order to make this so, we had to drop a fair number of features of the
- * other waitqueue code; notably:
+ * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all
+ * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher
+ * priority task a chance to run.
+ *
+ * Secondly, we had to drop a fair number of features of the other waitqueue
+ * code; notably:
 *
 *  - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue;
 *    all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right
@@ -24,12 +27,14 @@
 *  - the exclusive mode; because this requires preserving the list order
 *    and this is hard.
 *
- *  - custom wake functions; because you cannot give any guarantees about
- *    random code.
- *
- * As a side effect of this; the data structures are slimmer.
+ *  - custom wake callback functions; because you cannot give any guarantees
+ *    about random code. This also allows swait to be used in RT, such that
+ *    raw spinlock can be used for the swait queue head.
 *
- * One would recommend using this wait queue where possible.
+ * As a side effect of these; the data structures are slimmer albeit more ad-hoc.
+ * For all the above, note that simple wait queues should _only_ be used under
+ * very specific realtime constraints -- it is best to stick with the regular
+ * wait queues in most cases.
 */

struct task_struct;