On 10/25/21 10:03 AM, Marco Elver wrote:

On Mon, 25 Oct 2021 at 17:40, Jens Axboe [off-list ref] wrote:

quoted

On 10/25/21 8:29 AM, Marco Elver wrote:

quoted

On Mon, 25 Oct 2021 at 15:36, Jens Axboe [off-list ref] wrote:
[...]

quoted

quoted

write to 0xffffe8ffffd145b8 of 4 bytes by interrupt on cpu 1:
 sbitmap_queue_clear+0xca/0xf0 lib/sbitmap.c:606
 blk_mq_put_tag+0x82/0x90
 __blk_mq_free_request+0x114/0x180 block/blk-mq.c:507
 blk_mq_free_request+0x2c8/0x340 block/blk-mq.c:541
 __blk_mq_end_request+0x214/0x230 block/blk-mq.c:565
 blk_mq_end_request+0x37/0x50 block/blk-mq.c:574
 lo_complete_rq+0xca/0x170 drivers/block/loop.c:541
 blk_complete_reqs block/blk-mq.c:584 [inline]
 blk_done_softirq+0x69/0x90 block/blk-mq.c:589
 __do_softirq+0x12c/0x26e kernel/softirq.c:558
 run_ksoftirqd+0x13/0x20 kernel/softirq.c:920
 smpboot_thread_fn+0x22f/0x330 kernel/smpboot.c:164
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30

write to 0xffffe8ffffd145b8 of 4 bytes by interrupt on cpu 0:
 sbitmap_queue_clear+0xca/0xf0 lib/sbitmap.c:606
 blk_mq_put_tag+0x82/0x90
 __blk_mq_free_request+0x114/0x180 block/blk-mq.c:507
 blk_mq_free_request+0x2c8/0x340 block/blk-mq.c:541
 __blk_mq_end_request+0x214/0x230 block/blk-mq.c:565
 blk_mq_end_request+0x37/0x50 block/blk-mq.c:574
 lo_complete_rq+0xca/0x170 drivers/block/loop.c:541
 blk_complete_reqs block/blk-mq.c:584 [inline]
 blk_done_softirq+0x69/0x90 block/blk-mq.c:589
 __do_softirq+0x12c/0x26e kernel/softirq.c:558
 run_ksoftirqd+0x13/0x20 kernel/softirq.c:920
 smpboot_thread_fn+0x22f/0x330 kernel/smpboot.c:164
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30

This is just a per-cpu alloc hint, it's racy by nature. What's the
preferred way to silence these?

That was my guess, but couldn't quite say. We started looking at
write/write races as more likely to be harmful (vs. just read/write),
and are inclined to let syzbot send out more of such reports. Marking
intentional ones would be ideal so we'll be left with the
unintentional ones.

I would probably use WRITE_ONCE(), just to make sure the compiler
doesn't play games here; or if the code is entirely tolerant to even
the compiler miscompiling things, wrap the thing in data_race().

It's entirely tolerant, so something like this would do it?

Yup, looks reasonable,

Acked-by: Marco Elver <elver@google.com>

OK thanks, I'll queue it up for 5.16.

-- 
Jens Axboe