Author: kib
Date: Sun Jan 22 19:41:42 2017
New Revision: 312647
URL: https://svnweb.freebsd.org/changeset/base/312647
Log:
Add comments explaining unobvious td_critnest adjustments in
critical_exit().
Based on the discussion with: jhb
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential revision: D9276
MFC after: 1 week
Modified:
head/sys/kern/kern_switch.c
Modified: head/sys/kern/kern_switch.c
==============================================================================
--- head/sys/kern/kern_switch.c Sun Jan 22 19:38:45 2017 (r312646)
+++ head/sys/kern/kern_switch.c Sun Jan 22 19:41:42 2017 (r312647)
@@ -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--;
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