Re: [syzbot] KCSAN: data-race in flush_tlb_batched_pending / try_to_unmap_one

[Marco Elver]

On Thu, Nov 18, 2021 at 06:20AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    42eb8fdac2fc Merge tag 'gfs2-v5.16-rc2-fixes' of git://git..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13160026b00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a70237460d215073
> dashboard link: https://syzkaller.appspot.com/bug?extid=aa5bebed695edaccf0df
> 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+aa5bebed695edaccf0df@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> ==================================================================
> BUG: KCSAN: data-race in flush_tlb_batched_pending / try_to_unmap_one
> 
> write to 0xffff8881072cfbbc of 1 bytes by task 17406 on cpu 1:
>  flush_tlb_batched_pending+0x5f/0x80 mm/rmap.c:691
>  madvise_free_pte_range+0xee/0x7d0 mm/madvise.c:594
>  walk_pmd_range mm/pagewalk.c:128 [inline]
>  walk_pud_range mm/pagewalk.c:205 [inline]
>  walk_p4d_range mm/pagewalk.c:240 [inline]
>  walk_pgd_range mm/pagewalk.c:277 [inline]
>  __walk_page_range+0x981/0x1160 mm/pagewalk.c:379
>  walk_page_range+0x131/0x300 mm/pagewalk.c:475
>  madvise_free_single_vma mm/madvise.c:734 [inline]
>  madvise_dontneed_free mm/madvise.c:822 [inline]
>  madvise_vma mm/madvise.c:996 [inline]
>  do_madvise+0xe4a/0x1140 mm/madvise.c:1202
>  __do_sys_madvise mm/madvise.c:1228 [inline]
>  __se_sys_madvise mm/madvise.c:1226 [inline]
>  __x64_sys_madvise+0x5d/0x70 mm/madvise.c:1226
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> write to 0xffff8881072cfbbc of 1 bytes by task 71 on cpu 0:
>  set_tlb_ubc_flush_pending mm/rmap.c:636 [inline]
>  try_to_unmap_one+0x60e/0x1220 mm/rmap.c:1515
>  rmap_walk_anon+0x2fb/0x470 mm/rmap.c:2301
>  try_to_unmap+0xec/0x110
>  shrink_page_list+0xe91/0x2620 mm/vmscan.c:1719
>  shrink_inactive_list+0x3fb/0x730 mm/vmscan.c:2394
>  shrink_list mm/vmscan.c:2621 [inline]
>  shrink_lruvec+0x3c9/0x710 mm/vmscan.c:2940
>  shrink_node_memcgs+0x23e/0x410 mm/vmscan.c:3129
>  shrink_node+0x8f6/0x1190 mm/vmscan.c:3252
>  kswapd_shrink_node mm/vmscan.c:4022 [inline]
>  balance_pgdat+0x702/0xd30 mm/vmscan.c:4213
>  kswapd+0x200/0x340 mm/vmscan.c:4473
>  kthread+0x2c7/0x2e0 kernel/kthread.c:327
>  ret_from_fork+0x1f/0x30
> 
> value changed: 0x01 -> 0x00
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 71 Comm: kswapd0 Not tainted 5.16.0-rc1-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> ==================================================================

Reading 3ea277194daae, I can't quite determine if this is safe and
expected.

Per this observed write/write race, depending on interleaving
tlb_flush_batched can randomly be true or false after
set_tlb_ubc_flush_pending().

Is this safe?

Thanks,
-- Marco