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Add comments explaining unobvious td_critnest adjustments in

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critical_exit().

Based on the discussion with:	jhb
Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential revision:	D9276
MFC after:	1 week
This commit is contained in:
Konstantin Belousov 2017-01-22 19:41:42 +00:00
parent 25c6816845
commit 3467f88cd6
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=312647
1 changed files with 15 additions and 0 deletions
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15
sys/kern/kern_switch.c
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@ -206,7 +206,22 @@ critical_exit(void)
if (td->td_critnest == 1) {
td->td_critnest = 0;
/*
* Interrupt handlers execute critical_exit() on
* leave, and td_owepreempt may be left set by an
* interrupt handler only when td_critnest > 0. If we
* are decrementing td_critnest from 1 to 0, read
* td_owepreempt after decrementing, to not miss the
* preempt. Disallow compiler to reorder operations.
*/
__compiler_membar();
if (td->td_owepreempt && !kdb_active) {
/*
* Microoptimization: we committed to switch,
* disable preemption in interrupt handlers
* while spinning for the thread lock.
*/
td->td_critnest = 1;
thread_lock(td);
td->td_critnest--;