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[PATCH tip/core/rcu 15/16] scripts: Make checkpatch.pl warn on expedited RCU grace periods

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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The synchronize_rcu_expedited() and synchronize_sched_expedited()
expedited-grace-period primitives induce OS jitter, which can degrade
real-time response.  This commit therefore adds a checkpatch.pl warning
on any patch adding them.

Note that this patch does not warn on synchronize_srcu_expedited()
because it does not induce OS jitter, courtesy of its otherwise
much-maligned read-side memory barriers.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
---
 scripts/checkpatch.pl | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 90e1edc8dd42..0658a5f3029a 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -5011,6 +5011,7 @@ sub process {
 				     "memory barrier without comment\n" . $herecurr);
 			}
 		}
+
 # check for waitqueue_active without a comment.
 		if ($line =~ /\bwaitqueue_active\s*\(/) {
 			if (!ctx_has_comment($first_line, $linenr)) {
@@ -5018,6 +5019,24 @@ sub process {
 				     "waitqueue_active without comment\n" . $herecurr);
 			}
 		}
+
+# Check for expedited grace periods that interrupt non-idle non-nohz
+# online CPUs.  These expedited can therefore degrade real-time response
+# if used carelessly, and should be avoided where not absolutely
+# needed.  It is always OK to use synchronize_rcu_expedited() and
+# synchronize_sched_expedited() at boot time (before real-time applications
+# start) and in error situations where real-time response is compromised in
+# any case.  Note that synchronize_srcu_expedited() does -not- interrupt
+# other CPUs, so don't warn on uses of synchronize_srcu_expedited().
+# Of course, nothing comes for free, and srcu_read_lock() and
+# srcu_read_unlock() do contain full memory barriers in payment for
+# synchronize_srcu_expedited() non-interruption properties.
+		if ($line =~ /\b(synchronize_rcu_expedited|synchronize_sched_expedited)\(/) {
+			WARN("EXPEDITED_RCU_GRACE_PERIOD",
+			     "expedited RCU grace periods should be avoided in cases where they can degrade real-time response\n" . $herecurr);
+
+		}
+
 # check of hardware specific defines
 		if ($line =~ m@^.\s*\#\s*if.*\b(__i386__|__powerpc64__|__sun__|__s390x__)\b@ && $realfile !~ m@include/asm-@) {
 			CHK("ARCH_DEFINES",
-- 
1.8.1.5

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[PATCH tip/core/rcu 01/16] rcu: Drop RCU_USER_QS in favor of NO_HZ_FULL "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 15/16] scripts: Make checkpatch.pl warn on expedited RCU grace periods "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
    Re: [PATCH tip/core/rcu 15/16] scripts: Make checkpatch.pl warn on  expedited RCU grace periods Joe Perches <joe@perches.com> - 2015-07-18 00:40 +0200
  [PATCH tip/core/rcu 03/16] rcu: Change return type to bool "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 10/16] rcu: Make rcu_is_watching() really notrace "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 07/16] rcu: Fix obsolete priority-boosting comment "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 12/16] rcu: Fix backwards RCU_LOCKDEP_WARN() in synchronize_rcu_tasks() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 11/16] rcu: Rename rcu_lockdep_assert() to RCU_LOCKDEP_WARN() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 02/16] rcu: Deinline rcu_read_lock_sched_held() if DEBUG_LOCK_ALLOC "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 05/16] rcu: Hide RCU_NOCB_CPU behind RCU_EXPERT "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 06/16] rcu: Use WRITE_ONCE in RCU_INIT_POINTER "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 04/16] rcu: Add RCU-sched flavors of get-state and cond-sync "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 14/16] rcu: update MAINTAINERS entry "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 16/16] rcu: Don't disable CPU hotplug during OOM notifiers "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 09/16] cpu: Wait for RCU grace periods concurrently "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200
  [PATCH tip/core/rcu 13/16] rcu: Clarify CONFIG_RCU_EQS_DEBUG help text "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-18 00:30 +0200

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