[syzbot] KCSAN: data-race in sbitmap_queue_clear / sbitmap_queue_clear (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2f111a6fd5b5 Merge tag 'ceph-for-5.15-rc7' of git://github..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10dae330b00000
kernel config: https://syzkaller.appspot.com/x/.config?x=b2868748300e5cf6
dashboard link: https://syzkaller.appspot.com/bug?extid=4f8bfd804b4a1f95b8f6
compiler: Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+4f8bfd...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in sbitmap_queue_clear / sbitmap_queue_clear

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

value changed: 0x00000035 -> 0x00000044

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10 Comm: ksoftirqd/0 Not tainted 5.15.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

[Jens Axboe]

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

--
Jens Axboe

[Marco Elver]

On Mon, 25 Oct 2021 at 15:36, Jens Axboe <ax...@kernel.dk> wrote:
[...]

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().

[ A summary of a bunch of recommendations currently lives here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
]

Thanks,
-- Marco

[Jens Axboe]

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


diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index c6e2f1f2c4d2..2709ab825499 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -631,7 +631,7 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
static inline void sbitmap_update_cpu_hint(struct sbitmap *sb, int cpu, int tag)
{
if (likely(!sb->round_robin && tag < sb->depth))
- *per_cpu_ptr(sb->alloc_hint, cpu) = tag;
+ data_race(*per_cpu_ptr(sb->alloc_hint, cpu) = tag);
}

void sbitmap_queue_clear_batch(struct sbitmap_queue *sbq, int offset,

--
Jens Axboe

[Marco Elver]

Yup, looks reasonable,

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

[Jens Axboe]

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

--
Jens Axboe