This is a note to let you know that I've just added the patch titled
lib/group_cpus.c: avoid acquiring cpu hotplug lock in group_cpus_evenly
to the 6.1-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:
lib-group_cpus.c-avoid-acquiring-cpu-hotplug-lock-in.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit f07953806fd1f09054b8a7c16085bb0faaba9aec
Author: Ming Lei <ming.lei@xxxxxxxxxx>
Date: Mon Nov 20 16:35:59 2023 +0800
lib/group_cpus.c: avoid acquiring cpu hotplug lock in group_cpus_evenly
[ Upstream commit 0263f92fadbb9d294d5971ac57743f882c93b2b3 ]
group_cpus_evenly() could be part of storage driver's error handler, such
as nvme driver, when may happen during CPU hotplug, in which storage queue
has to drain its pending IOs because all CPUs associated with the queue
are offline and the queue is becoming inactive. And handling IO needs
error handler to provide forward progress.
Then deadlock is caused:
1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's
handler is waiting for inflight IO
2) error handler is waiting for CPU hotplug lock
3) inflight IO can't be completed in blk-mq's CPU hotplug handler
because error handling can't provide forward progress.
Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(),
in which two stage spreads are taken: 1) the 1st stage is over all present
CPUs; 2) the end stage is over all other CPUs.
Turns out the two stage spread just needs consistent 'cpu_present_mask',
and remove the CPU hotplug lock by storing it into one local cache. This
way doesn't change correctness, because all CPUs are still covered.
Link: https://lkml.kernel.org/r/20231120083559.285174-1-ming.lei@xxxxxxxxxx
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Reported-by: Yi Zhang <yi.zhang@xxxxxxxxxx>
Reported-by: Guangwu Zhang <guazhang@xxxxxxxxxx>
Tested-by: Guangwu Zhang <guazhang@xxxxxxxxxx>
Reviewed-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
Cc: Keith Busch <kbusch@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/lib/group_cpus.c b/lib/group_cpus.c
index 99f08c6cb9d97..156b1446d2a20 100644
--- a/lib/group_cpus.c
+++ b/lib/group_cpus.c
@@ -365,13 +365,25 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
if (!masks)
goto fail_node_to_cpumask;
- /* Stabilize the cpumasks */
- cpus_read_lock();
build_node_to_cpumask(node_to_cpumask);
+ /*
+ * Make a local cache of 'cpu_present_mask', so the two stages
+ * spread can observe consistent 'cpu_present_mask' without holding
+ * cpu hotplug lock, then we can reduce deadlock risk with cpu
+ * hotplug code.
+ *
+ * Here CPU hotplug may happen when reading `cpu_present_mask`, and
+ * we can live with the case because it only affects that hotplug
+ * CPU is handled in the 1st or 2nd stage, and either way is correct
+ * from API user viewpoint since 2-stage spread is sort of
+ * optimization.
+ */
+ cpumask_copy(npresmsk, data_race(cpu_present_mask));
+
/* grouping present CPUs first */
ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
- cpu_present_mask, nmsk, masks);
+ npresmsk, nmsk, masks);
if (ret < 0)
goto fail_build_affinity;
nr_present = ret;
@@ -386,15 +398,13 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
curgrp = 0;
else
curgrp = nr_present;
- cpumask_andnot(npresmsk, cpu_possible_mask, cpu_present_mask);
+ cpumask_andnot(npresmsk, cpu_possible_mask, npresmsk);
ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask,
npresmsk, nmsk, masks);
if (ret >= 0)
nr_others = ret;
fail_build_affinity:
- cpus_read_unlock();
-
if (ret >= 0)
WARN_ON(nr_present + nr_others < numgrps);
