linux-kernel - Re: [PATCH] firmware: wake all waiters

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Date:   Thu, 29 Jun 2017 12:08:54 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        DanielWagnerwagi@...om.org, Boqun Feng <boqun.feng@...il.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Ming Lei <ming.lei@...hat.com>,
        "Li, Yi" <yi1.li@...ux.intel.com>,
        "AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        David Howells <dhowells@...hat.com>,
        Alan Cox <alan@...ux.intel.com>, Theodore Ts'o <tytso@....edu>,
        oss-drivers@...ronome.com
Subject: Re: [PATCH] firmware: wake all waiters

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;