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| author | Konstantin Belousov <kib@FreeBSD.org> | 2017-01-22 19:41:42 +0000 |
|---|---|---|
| committer | Konstantin Belousov <kib@FreeBSD.org> | 2017-01-22 19:41:42 +0000 |
| commit | 3467f88cd67c19360f4e8662e6f35701aa4921ce (patch) | |
| tree | df2a3c0988b663e8e633cab2a42d61847d672404 /sys/kern | |
| parent | 25c6816845534edfc308efb67a2efa943b87dfc8 (diff) | |
| download | src-3467f88cd67c19360f4e8662e6f35701aa4921ce.tar.gz src-3467f88cd67c19360f4e8662e6f35701aa4921ce.zip | |
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
Notes
Notes:
svn path=/head/; revision=312647
Diffstat (limited to 'sys/kern')
| -rw-r--r-- | sys/kern/kern_switch.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/sys/kern/kern_switch.c b/sys/kern/kern_switch.c index d0009b1042f3..f58acd5ed4c6 100644 --- a/sys/kern/kern_switch.c +++ b/sys/kern/kern_switch.c @@ -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--; |