When power management for SCSI is enabled and if a device uses blk-mq,
it is possible to trigger a `NULL` pointer exception when resuming that
device. The NPE is triggered when trying to dereference the `request_fn`
function pointer of the device's `request_queue`:

    __blk_run_queue_uncond:470
    __blk_run_queue:490
    blk_post_runtime_resume:3889
    sdev_runtime_resume:263
    scsi_runtime_resume:275

When the SCSI device is being allocated by `scsi_alloc_sdev`, the
device's request queue will either be initialized via
`scsi_mq_alloc_queue` or `scsi_old_alloc_queue`. But the `request_fn`
member of the request queue is in fact only being set in
`scsi_old_alloc_queue`, which will then later cause the mentioned NPE.

Fix the issue by checking whether the `request_fn` is set in
`__blk_run_queue_uncond`. In case it is unset, we'll silently return and
not try to invoke the callback, thus fixing the NPE.

Signed-off-by: Patrick Steinhardt <redacted>
---

Since at least v4.14, I am easily able to trigger above NPE by
unplugging USB mass storage devices on my computer (Skylake, ASUS
Z170I) with CONFIG_SCSI_MQ_DEFAULT=y. The attached patch fixes
the issue, but keep in mind that this is my first patch, so the
proposed fix may not be appropriate at all. Feedback would be
highly appreciated.

 block/blk-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index f84a9b7b6f5a..0a2041660cd9 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -456,7 +456,7 @@ inline void __blk_run_queue_uncond(struct request_queue *q)
        lockdep_assert_held(q->queue_lock);
        WARN_ON_ONCE(q->mq_ops);

-       if (unlikely(blk_queue_dead(q)))
+       if (unlikely(!q->request_fn) || unlikely(blk_queue_dead(q)))
                return;