Merge branch 'linus' into locking/core, to resolve conflicts

Conflicts:
	include/linux/compiler-clang.h
	include/linux/compiler-gcc.h
	include/linux/compiler-intel.h
	include/uapi/linux/stddef.h

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 17 insertions, 11 deletions

diff --git a/include/linux/swait.h b/include/linux/swait.h
index 73e97a08d3d0..c98aaf677466 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 #ifndef _LINUX_SWAIT_H
 #define _LINUX_SWAIT_H
 
@@ -9,13 +10,16 @@
 /*
  * 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 +28,14 @@
  *  - 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;