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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
This commit is contained in:
Linus Torvalds 2017-10-27 17:17:25 -07:00
parent 02c7dfc006 88796e7e5c
commit 11dc76f0a4
1 changed files with 16 additions and 11 deletions
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27
include/linux/swait.h
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@ -9,13 +9,16 @@
/* /*
* Simple wait queues * Simple wait queues
* *
* While these are very similar to the other/complex wait queues (wait.h) the * While these are very similar to regular wait queues (wait.h) the most
* most important difference is that the simple waitqueue allows for * important difference is that the simple waitqueue allows for deterministic
* deterministic behaviour -- IOW it has strictly bounded IRQ and lock hold * behaviour -- IOW it has strictly bounded IRQ and lock hold times.
* times.
* *
* In order to make this so, we had to drop a fair number of features of the * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all
* other waitqueue code; notably: * 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; * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue;
* all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right * 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 * - the exclusive mode; because this requires preserving the list order
* and this is hard. * and this is hard.
* *
* - custom wake functions; because you cannot give any guarantees about * - custom wake callback functions; because you cannot give any guarantees
* random code. * about random code. This also allows swait to be used in RT, such that
* raw spinlock can be used for the swait queue head.
* *
* As a side effect of this; the data structures are slimmer. * 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
* One would recommend using this wait queue where possible. * very specific realtime constraints -- it is best to stick with the regular
* wait queues in most cases.
*/ */
struct task_struct; struct task_struct;