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Date:   Mon, 10 Aug 2020 13:45:16 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     syzbot <syzbot+0d4522639ba75b02bf19@syzkaller.appspotmail.com>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, syzkaller-bugs@googlegroups.com
Subject: Re: KCSAN: data-race in __xa_clear_mark / xas_find_marked
Message-ID: <20200810124516.GM17456@casper.infradead.org>
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On Mon, Aug 10, 2020 at 05:41:18AM -0700, syzbot wrote:
> ==================================================================
> BUG: KCSAN: data-race in __xa_clear_mark / xas_find_marked

This is not a bug.  xas_find_marked() is well aware that it is only
running under the RCU lock and may see marks which are concurrently
being set or cleared.

What's the latest wart to add so that this stupid checker will shut up?

> write to 0xffff8880bace9b30 of 8 bytes by interrupt on cpu 1:
>  instrument_write include/linux/instrumented.h:42 [inline]
>  __test_and_clear_bit include/asm-generic/bitops/instrumented-non-atomic.h:85 [inline]
>  node_clear_mark lib/xarray.c:100 [inline]
>  xas_clear_mark lib/xarray.c:908 [inline]
>  __xa_clear_mark+0x229/0x350 lib/xarray.c:1726
>  test_clear_page_writeback+0x28d/0x480 mm/page-writeback.c:2739
>  end_page_writeback+0xa7/0x110 mm/filemap.c:1369
>  page_endio+0x1aa/0x1e0 mm/filemap.c:1400
>  mpage_end_io+0x186/0x1d0 fs/mpage.c:54
>  bio_endio+0x28a/0x370 block/bio.c:1447
>  req_bio_endio block/blk-core.c:259 [inline]
>  blk_update_request+0x535/0xbd0 block/blk-core.c:1576
>  blk_mq_end_request+0x22/0x50 block/blk-mq.c:562
>  lo_complete_rq+0xca/0x180 drivers/block/loop.c:500
>  blk_done_softirq+0x1a5/0x200 block/blk-mq.c:586
>  __do_softirq+0x198/0x360 kernel/softirq.c:298
>  run_ksoftirqd+0x2f/0x60 kernel/softirq.c:652
>  smpboot_thread_fn+0x347/0x530 kernel/smpboot.c:165
>  kthread+0x20d/0x230 kernel/kthread.c:292
>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> 
> read to 0xffff8880bace9b30 of 8 bytes by task 12715 on cpu 0:
>  xas_find_chunk include/linux/xarray.h:1625 [inline]
>  xas_find_marked+0x22f/0x6b0 lib/xarray.c:1198
>  find_get_pages_range_tag+0xa3/0x580 mm/filemap.c:1976
>  pagevec_lookup_range_tag+0x37/0x50 mm/swap.c:1120
>  __filemap_fdatawait_range+0xab/0x1b0 mm/filemap.c:519
>  filemap_fdatawait_range mm/filemap.c:554 [inline]
>  filemap_write_and_wait_range+0x119/0x2a0 mm/filemap.c:664
>  generic_file_read_iter+0x11d/0x3e0 mm/filemap.c:2375
>  call_read_iter include/linux/fs.h:1866 [inline]
>  generic_file_splice_read+0x22b/0x310 fs/splice.c:312
>  do_splice_to fs/splice.c:870 [inline]
>  splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950
>  do_splice_direct+0xf2/0x170 fs/splice.c:1059
>  do_sendfile+0x56a/0xba0 fs/read_write.c:1540
>  __do_sys_sendfile64 fs/read_write.c:1595 [inline]
>  __se_sys_sendfile64 fs/read_write.c:1587 [inline]
>  __x64_sys_sendfile64+0xa9/0x130 fs/read_write.c:1587
>  do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 12715 Comm: syz-executor.4 Not tainted 5.8.0-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> ==================================================================
> 
> 
> ---
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> 
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>