[PATCH 1/8] membarrier: Document why membarrier() works

[Andy Lutomirski]

We had a nice comment at the top of membarrier.c explaining why membarrier
worked in a handful of scenarios, but that consisted more of a list of
things not to forget than an actual description of the algorithm and why it
should be expected to work.

Add a comment explaining my understanding of the algorithm.  This exposes a
couple of implementation issues that I will hopefully fix up in subsequent
patches.

Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 kernel/sched/membarrier.c | 55 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index b5add64d9698..3173b063d358 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -7,6 +7,61 @@
 #include "sched.h"
 
 /*
+ * The basic principle behind the regular memory barrier mode of membarrier()
+ * is as follows.  For each CPU, membarrier() operates in one of two
+ * modes.  Either it sends an IPI or it does not. If membarrier() sends an
+ * IPI, then we have the following sequence of events:
+ *
+ * 1. membarrier() does smp_mb().
+ * 2. membarrier() does a store (the IPI request payload) that is observed by
+ *    the target CPU.
+ * 3. The target CPU does smp_mb().
+ * 4. The target CPU does a store (the completion indication) that is observed
+ *    by membarrier()'s wait-for-IPIs-to-finish request.
+ * 5. membarrier() does smp_mb().
+ *
+ * So all pre-membarrier() local accesses are visible after the IPI on the
+ * target CPU and all pre-IPI remote accesses are visible after
+ * membarrier(). IOW membarrier() has synchronized both ways with the target
+ * CPU.
+ *
+ * (This has the caveat that membarrier() does not interrupt the CPU that it's
+ * running on at the time it sends the IPIs. However, if that is the CPU on
+ * which membarrier() starts and/or finishes, membarrier() does smp_mb() and,
+ * if not, then membarrier() scheduled, and scheduling had better include a
+ * full barrier somewhere for basic correctness regardless of membarrier.)
+ *
+ * If membarrier() does not send an IPI, this means that membarrier() reads
+ * cpu_rq(cpu)->curr->mm and that the result is not equal to the target
+ * mm.  Let's assume for now that tasks never change their mm field.  The
+ * sequence of events is:
+ *
+ * 1. Target CPU switches away from the target mm (or goes lazy or has never
+ *    run the target mm in the first place). This involves smp_mb() followed
+ *    by a write to cpu_rq(cpu)->curr.
+ * 2. membarrier() does smp_mb(). (This is NOT synchronized with any action
+ *    done by the target.)
+ * 3. membarrier() observes the value written in step 1 and does *not* observe
+ *    the value written in step 5.
+ * 4. membarrier() does smp_mb().
+ * 5. Target CPU switches back to the target mm and writes to
+ *    cpu_rq(cpu)->curr. (This is NOT synchronized with any action on
+ *    membarrier()'s part.)
+ * 6. Target CPU executes smp_mb()
+ *
+ * All pre-schedule accesses on the remote CPU are visible after membarrier()
+ * because they all precede the target's write in step 1 and are synchronized
+ * to the local CPU by steps 3 and 4.  All pre-membarrier() accesses on the
+ * local CPU are visible on the remote CPU after scheduling because they
+ * happen before the smp_mb(); read in steps 2 and 3 and that read preceeds
+ * the target's smp_mb() in step 6.
+ *
+ * However, tasks can change their ->mm, e.g., via kthread_use_mm().  So
+ * tasks that switch their ->mm must follow the same rules as the scheduler
+ * changing rq->curr, and the membarrier() code needs to do both dereferences
+ * carefully.
+ *
+ *
  * For documentation purposes, here are some membarrier ordering
  * scenarios to keep in mind:
  *
-- 
2.31.1