linux-kernel - [PATCH RT 3/4] tasklet: Fix UP case for tasklet CHAINED state

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Message-Id: <2262ed0571ef9c2f24752189b55c965ce9c16d2f.1591898986.git.zanussi@kernel.org>
Date:   Thu, 11 Jun 2020 13:10:16 -0500
From:   zanussi@...nel.org
To:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <wagi@...om.org>,
        Clark Williams <williams@...hat.com>,
        Pavel Machek <pavel@...x.de>, Tom Zanussi <zanussi@...nel.org>
Subject: [PATCH RT 3/4] tasklet: Fix UP case for tasklet CHAINED state

From: Tom Zanussi <zanussi@...nel.org>

v4.19.127-rt55-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


commit 62d0a2a30cd0 (tasklet: Address a race resulting in
double-enqueue) addresses a problem that can result in a tasklet being
enqueued on two cpus at the same time by combining the RUN flag with a
new CHAINED flag, and relies on the combination to be present in order
to zero it out, which can never happen on (!SMP and !PREEMPT_RT_FULL)
because the RUN flag is SMP/PREEMPT_RT_FULL-only.

So make sure the above commit is only applied for the SMP ||
PREEMPT_RT_FULL case.

Fixes: 62d0a2a30cd0 ("tasklet: Address a race resulting in double-enqueue")
Signed-off-by: Tom Zanussi <zanussi@...nel.org>
Reported-by: Ramon Fried <rfried.dev@...il.com>
Tested-By: Ramon Fried <rfried.dev@...il.com>
---
 kernel/softirq.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 73dae64bfc9c..9bad7a16dc61 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -947,10 +947,12 @@ static void __tasklet_schedule_common(struct tasklet_struct *t,
 	 * is locked before adding it to the list.
 	 */
 	if (test_bit(TASKLET_STATE_SCHED, &t->state)) {
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
 		if (test_and_set_bit(TASKLET_STATE_CHAINED, &t->state)) {
 			tasklet_unlock(t);
 			return;
 		}
+#endif
 		t->next = NULL;
 		*head->tail = t;
 		head->tail = &(t->next);
@@ -1044,7 +1046,11 @@ static void tasklet_action_common(struct softirq_action *a,
 again:
 		t->func(t->data);
 
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT_FULL)
 		while (cmpxchg(&t->state, TASKLET_STATEF_RC, 0) != TASKLET_STATEF_RC) {
+#else
+		while (!tasklet_tryunlock(t)) {
+#endif
 			/*
 			 * If it got disabled meanwhile, bail out:
 			 */
-- 
2.17.1