lib/group_cpus.c: avoid acquiring cpu hotplug lock in group_cpus_evenly

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/[email protected]
Signed-off-by: Ming Lei <[email protected]>
Reported-by: Yi Zhang <[email protected]>
Reported-by: Guangwu Zhang <[email protected]>
Tested-by: Guangwu Zhang <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Reviewed-by: Jens Axboe <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

lib/group_cpus.c

@@ -366,13 +366,25 @@ 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;
@@ -387,15 +399,13 @@ 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);