Re: Critical section

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Wed, 26 Nov 2014 16:41:01 +0900
   From: Masao Uebayashi <uebayasi%gmail.com@localhost>

   The problem of kpreempt_*() API is that its meaning is overriden by
   kernel internal (scheduler, sync primitives, ...).  This change
   separates the internal use (scheduler disables preeemption) and
   others (kernel subsystem code executes critical section).  Detect
   sleep from within critical section in mi_switch().

   The only problem I've seen is, cprng_fast.c calling percpu_getref() in
   KASSERT(); it's kind of re-entrance.

Generally sounds reasonable to me.  I'm not happy about prohibiting
nesting, though, and I don't think `critical section' is the right
word for it, though -- it's too broad.

Are there any uses of kpreempt_disable/kpreempt_enable outside the
scheduler that intentionally allow voluntary switching?  Are there any
uses of KPREEMPT_DISABLE/KPREEMPT_ENABLE inside the scheduler that
require nesting?

If the answer to both is `no', perhaps we could make KPREEMPT_DISABLE
set a bit, and make kpreempt_disable increment a counter, and make
mi_switch kassert that the bit is set and the counter is zero.