From 25595eb6aaa9fbb31330f1e0b400642694bc6574 Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Tue, 20 Oct 2020 09:47:15 -0400 Subject: [PATCH] sched: membarrier: document memory ordering scenarios Document membarrier ordering scenarios in membarrier.c. Thanks to Alan Stern for refreshing my memory. Now that I have those in mind, it seems appropriate to serialize them to comments for posterity. Signed-off-by: Mathieu Desnoyers Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20201020134715.13909-4-mathieu.desnoyers@efficios.com --- kernel/sched/membarrier.c | 128 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 128 insertions(+) diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c index f223f3590b8f..5a40b3828ff2 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -6,6 +6,134 @@ */ #include "sched.h" +/* + * For documentation purposes, here are some membarrier ordering + * scenarios to keep in mind: + * + * A) Userspace thread execution after IPI vs membarrier's memory + * barrier before sending the IPI + * + * Userspace variables: + * + * int x = 0, y = 0; + * + * The memory barrier at the start of membarrier() on CPU0 is necessary in + * order to enforce the guarantee that any writes occurring on CPU0 before + * the membarrier() is executed will be visible to any code executing on + * CPU1 after the IPI-induced memory barrier: + * + * CPU0 CPU1 + * + * x = 1 + * membarrier(): + * a: smp_mb() + * b: send IPI IPI-induced mb + * c: smp_mb() + * r2 = y + * y = 1 + * barrier() + * r1 = x + * + * BUG_ON(r1 == 0 && r2 == 0) + * + * The write to y and load from x by CPU1 are unordered by the hardware, + * so it's possible to have "r1 = x" reordered before "y = 1" at any + * point after (b). If the memory barrier at (a) is omitted, then "x = 1" + * can be reordered after (a) (although not after (c)), so we get r1 == 0 + * and r2 == 0. This violates the guarantee that membarrier() is + * supposed by provide. + * + * The timing of the memory barrier at (a) has to ensure that it executes + * before the IPI-induced memory barrier on CPU1. + * + * B) Userspace thread execution before IPI vs membarrier's memory + * barrier after completing the IPI + * + * Userspace variables: + * + * int x = 0, y = 0; + * + * The memory barrier at the end of membarrier() on CPU0 is necessary in + * order to enforce the guarantee that any writes occurring on CPU1 before + * the membarrier() is executed will be visible to any code executing on + * CPU0 after the membarrier(): + * + * CPU0 CPU1 + * + * x = 1 + * barrier() + * y = 1 + * r2 = y + * membarrier(): + * a: smp_mb() + * b: send IPI IPI-induced mb + * c: smp_mb() + * r1 = x + * BUG_ON(r1 == 0 && r2 == 1) + * + * The writes to x and y are unordered by the hardware, so it's possible to + * have "r2 = 1" even though the write to x doesn't execute until (b). If + * the memory barrier at (c) is omitted then "r1 = x" can be reordered + * before (b) (although not before (a)), so we get "r1 = 0". This violates + * the guarantee that membarrier() is supposed to provide. + * + * The timing of the memory barrier at (c) has to ensure that it executes + * after the IPI-induced memory barrier on CPU1. + * + * C) Scheduling userspace thread -> kthread -> userspace thread vs membarrier + * + * CPU0 CPU1 + * + * membarrier(): + * a: smp_mb() + * d: switch to kthread (includes mb) + * b: read rq->curr->mm == NULL + * e: switch to user (includes mb) + * c: smp_mb() + * + * Using the scenario from (A), we can show that (a) needs to be paired + * with (e). Using the scenario from (B), we can show that (c) needs to + * be paired with (d). + * + * D) exit_mm vs membarrier + * + * Two thread groups are created, A and B. Thread group B is created by + * issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD. + * Let's assume we have a single thread within each thread group (Thread A + * and Thread B). Thread A runs on CPU0, Thread B runs on CPU1. + * + * CPU0 CPU1 + * + * membarrier(): + * a: smp_mb() + * exit_mm(): + * d: smp_mb() + * e: current->mm = NULL + * b: read rq->curr->mm == NULL + * c: smp_mb() + * + * Using scenario (B), we can show that (c) needs to be paired with (d). + * + * E) kthread_{use,unuse}_mm vs membarrier + * + * CPU0 CPU1 + * + * membarrier(): + * a: smp_mb() + * kthread_unuse_mm() + * d: smp_mb() + * e: current->mm = NULL + * b: read rq->curr->mm == NULL + * kthread_use_mm() + * f: current->mm = mm + * g: smp_mb() + * c: smp_mb() + * + * Using the scenario from (A), we can show that (a) needs to be paired + * with (g). Using the scenario from (B), we can show that (c) needs to + * be paired with (d). + */ + /* * Bitmask made from a "or" of all commands within enum membarrier_cmd, * except MEMBARRIER_CMD_QUERY.