Patch "workqueue: Make sure that wq_unbound_cpumask is never empty" has been added to the 6.6-stable tree

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This is a note to let you know that I've just added the patch titled

    workqueue: Make sure that wq_unbound_cpumask is never empty

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     workqueue-make-sure-that-wq_unbound_cpumask-is-never-empty.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 4a6c5607d4502ccd1b15b57d57f17d12b6f257a7 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Tue, 21 Nov 2023 11:39:36 -1000
Subject: workqueue: Make sure that wq_unbound_cpumask is never empty

From: Tejun Heo <tj@xxxxxxxxxx>

commit 4a6c5607d4502ccd1b15b57d57f17d12b6f257a7 upstream.

During boot, depending on how the housekeeping and workqueue.unbound_cpus
masks are set, wq_unbound_cpumask can end up empty. Since 8639ecebc9b1
("workqueue: Implement non-strict affinity scope for unbound workqueues"),
this may end up feeding -1 as a CPU number into scheduler leading to oopses.

  BUG: unable to handle page fault for address: ffffffff8305e9c0
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  ...
  Call Trace:
   <TASK>
   select_idle_sibling+0x79/0xaf0
   select_task_rq_fair+0x1cb/0x7b0
   try_to_wake_up+0x29c/0x5c0
   wake_up_process+0x19/0x20
   kick_pool+0x5e/0xb0
   __queue_work+0x119/0x430
   queue_work_on+0x29/0x30
  ...

An empty wq_unbound_cpumask is a clear misconfiguration and already
disallowed once system is booted up. Let's warn on and ignore
unbound_cpumask restrictions which lead to no unbound cpus. While at it,
also remove now unncessary empty check on wq_unbound_cpumask in
wq_select_unbound_cpu().

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-and-Tested-by: Yong He <alexyonghe@xxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20231120121623.119780-1-alexyonghe@xxxxxxxxxxx
Fixes: 8639ecebc9b1 ("workqueue: Implement non-strict affinity scope for unbound workqueues")
Cc: stable@xxxxxxxxxxxxxxx # v6.6+
Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/workqueue.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 6e578f576a6f..2989b57e154a 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1684,9 +1684,6 @@ static int wq_select_unbound_cpu(int cpu)
 		pr_warn_once("workqueue: round-robin CPU selection forced, expect performance impact\n");
 	}
 
-	if (cpumask_empty(wq_unbound_cpumask))
-		return cpu;
-
 	new_cpu = __this_cpu_read(wq_rr_cpu_last);
 	new_cpu = cpumask_next_and(new_cpu, wq_unbound_cpumask, cpu_online_mask);
 	if (unlikely(new_cpu >= nr_cpu_ids)) {
@@ -6515,6 +6512,17 @@ static inline void wq_watchdog_init(void) { }
 
 #endif	/* CONFIG_WQ_WATCHDOG */
 
+static void __init restrict_unbound_cpumask(const char *name, const struct cpumask *mask)
+{
+	if (!cpumask_intersects(wq_unbound_cpumask, mask)) {
+		pr_warn("workqueue: Restricting unbound_cpumask (%*pb) with %s (%*pb) leaves no CPU, ignoring\n",
+			cpumask_pr_args(wq_unbound_cpumask), name, cpumask_pr_args(mask));
+		return;
+	}
+
+	cpumask_and(wq_unbound_cpumask, wq_unbound_cpumask, mask);
+}
+
 /**
  * workqueue_init_early - early init for workqueue subsystem
  *
@@ -6534,11 +6542,11 @@ void __init workqueue_init_early(void)
 	BUILD_BUG_ON(__alignof__(struct pool_workqueue) < __alignof__(long long));
 
 	BUG_ON(!alloc_cpumask_var(&wq_unbound_cpumask, GFP_KERNEL));
-	cpumask_copy(wq_unbound_cpumask, housekeeping_cpumask(HK_TYPE_WQ));
-	cpumask_and(wq_unbound_cpumask, wq_unbound_cpumask, housekeeping_cpumask(HK_TYPE_DOMAIN));
-
+	cpumask_copy(wq_unbound_cpumask, cpu_possible_mask);
+	restrict_unbound_cpumask("HK_TYPE_WQ", housekeeping_cpumask(HK_TYPE_WQ));
+	restrict_unbound_cpumask("HK_TYPE_DOMAIN", housekeeping_cpumask(HK_TYPE_DOMAIN));
 	if (!cpumask_empty(&wq_cmdline_cpumask))
-		cpumask_and(wq_unbound_cpumask, wq_unbound_cpumask, &wq_cmdline_cpumask);
+		restrict_unbound_cpumask("workqueue.unbound_cpus", &wq_cmdline_cpumask);
 
 	pwq_cache = KMEM_CACHE(pool_workqueue, SLAB_PANIC);
 
-- 
2.43.0



Patches currently in stable-queue which might be from tj@xxxxxxxxxx are

queue-6.6/cgroup_freezer-cgroup_freezing-check-if-not-frozen.patch
queue-6.6/workqueue-make-sure-that-wq_unbound_cpumask-is-never-empty.patch




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