From 405c00e43a9c72a0b9b318cc2d2e298386ffb58e Mon Sep 17 00:00:00 2001 From: DAI RENJIE Date: Fri, 21 Aug 2026 14:08:17 +0000 Subject: [PATCH] resource: fix lost wakeup when waiting for a muxed region A task waiting for a muxed region can sleep forever in TASK_UNINTERRUPTIBLE even though the region it waits for is already free. __request_region_locked() queues itself on muxed_resource_wait and drops resource_lock before setting TASK_UNINTERRUPTIBLE, while __release_region() wakes the queue after dropping the same lock. A wakeup landing in between finds TASK_RUNNING, does not match TASK_NORMAL and is discarded; callers hold a muxed region only across a bounded transaction, so no further release is coming. The task is unkillable and its caller never returns. The window is one store wide, but an interrupt is enough to hold the waiter in it, and the machine this was seen on runs PREEMPT_DYNAMIC in its voluntary default. Since v6.11 spd5118 exports the DDR5 sensors of AMD boards through i2c-piix4, which takes a muxed region per SMBus transaction; a third of the in-tree users of request_muxed_region() are hwmon drivers, so reading a world-readable attribute is all an unprivileged user needs to drive the contention. The blocked task sleeps holding the i2c adapter bus lock, and 27 more piled up behind it. Reproduced by building a kernel with the two orderings selectable at runtime and a 2ms delay inside the window. Switching only that knob, a two-thread barriered reproducer loses the wakeup 200 times out of 200 before the fix and 0 out of 200 after it; without the delay it goes 20000 times through the wait path and loses none. Fix it by setting the task state before dropping resource_lock, as prepare_to_wait() does: the releasing side needs resource_lock to unlink the resource, so it cannot reach the wakeup before the state is published. Link: https://lore.kernel.org/20260821-b4-resource-muxed-lost-wakeup-v1-1-37eb6473a76c@gmail.com Fixes: 8b6d043b7ee2 ("resource: shared I/O region support") Signed-off-by: DAI RENJIE Signed-off-by: Andrew Morton Reviewed-by: Bradley Morgan Reviewed-by: Andrew Morton Assisted-by: Claude:claude-opus-5 Cc: Mark Brown Cc: Kees Cook Cc: Bjorn Helgaas Cc: --- kernel/resource.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/resource.c b/kernel/resource.c index e60539a55541..54d7199695fb 100644 --- a/kernel/resource.c +++ b/kernel/resource.c @@ -1350,8 +1350,8 @@ static int __request_region_locked(struct resource *res, struct resource *parent } if (conflict->flags & flags & IORESOURCE_MUXED) { add_wait_queue(&muxed_resource_wait, &wait); - write_unlock(&resource_lock); set_current_state(TASK_UNINTERRUPTIBLE); + write_unlock(&resource_lock); schedule(); remove_wait_queue(&muxed_resource_wait, &wait); write_lock(&resource_lock);