|author||Linus Torvalds <firstname.lastname@example.org>||2017-10-27 17:17:25 -0700|
|committer||Linus Torvalds <email@example.com>||2017-10-27 17:17:25 -0700|
Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull scheduler fix from Ingo Molnar: "Update the <linux/swait.h> documentation to discourage their use" * 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: sched/swait: Document it clearly that the swait facilities are special and shouldn't be used
1 files changed, 16 insertions, 11 deletions
diff --git a/include/linux/swait.h b/include/linux/swait.h
index 73e97a08d3d0..cf30f5022472 100644
@@ -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.