[PATCH 2/2] sched_ext: Protect the idle-search scratch nodemask with irqsave

[Wanwu Li]

pick_idle_cpu_from_online_nodes() uses the per-CPU per_cpu_unvisited
nodemask as scratch while walking the online nodes, protected only by
preempt_disable(). preempt_disable() does not mask IRQs and the idle
kfuncs are callable from IRQ-enabled contexts, so a nested invocation
on the same CPU can overwrite the mask with nodes_copy() while the
interrupted invocation is still iterating it, leading to a wrong node
traversal and a wrong idle CPU pick.

Switch to irqsave so a nested invocation can't run on the same CPU.
A stack-allocated nodemask would also close the race, but that would
enlarge the diff to fix a race that is already rare (per-node idle,
CONFIG_NUMA and a cross-node search all at once); irqsave is the
minimal fix for the context that actually triggers it. The NMI case is
deliberately not addressed: there is no legitimate reason to
call pick_idle from NMI and doing so poses no crash risk, so such a
caller is on its own.

Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Link: https://lore.kernel.org/r/d84b31727f04e1ed0d40042ba1c09e61@xxxxxxxxxx
Signed-off-by: Wanwu Li <liwanwu@xxxxxxxxxx>
---
 kernel/sched/ext/idle.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext/idle.c b/kernel/sched/ext/idle.c
index d2973fb3af6d..93e2e0b2d1f8 100644
--- a/kernel/sched/ext/idle.c
+++ b/kernel/sched/ext/idle.c
@@ -153,7 +153,18 @@ static s32 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, i
 	nodemask_t *unvisited;
 	s32 cpu = -EBUSY;
 
-	preempt_disable();
+	/*
+	 * @per_cpu_unvisited is per-CPU scratch and the idle kfuncs can be
+	 * called from IRQ-enabled contexts, so mask IRQs to keep a nested
+	 * invocation from clobbering the mask an outer invocation is still
+	 * iterating.
+	 *
+	 * NMI nesting is not handled: there is no legitimate reason to call
+	 * pick_idle from NMI and doing so poses no crash risk, so such a
+	 * caller is on its own.
+	 */
+	guard(irqsave)();
+
 	unvisited = this_cpu_ptr(&per_cpu_unvisited);
 
 	/*
@@ -183,7 +194,6 @@ static s32 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, i
 		if (cpu >= 0)
 			break;
 	}
-	preempt_enable();
 
 	return cpu;
 }
-- 
2.25.1