Re: Kernel Concurrency Sanitizer (KCSAN)

[Marco Elver]

Hi Daniel,

On Tue, 1 Oct 2019 at 16:50, Daniel Axtens <dja@xxxxxxxxxx> wrote:
>
> Hi Marco,
>
> > We would like to share a new data-race detector for the Linux kernel:
> > Kernel Concurrency Sanitizer (KCSAN) --
> > https://github.com/google/ktsan/wiki/KCSAN  (Details:
> > https://github.com/google/ktsan/blob/kcsan/Documentation/dev-tools/kcsan.rst)
>
> This builds and begins to boot on powerpc, which is fantastic.
>
> I'm seeing a lot of reports for locks are changed while being watched by
> kcsan, so many that it floods the console and stalls the boot.
>
> I think, if I've understood correctly, that this is because powerpc
> doesn't use the queued lock implementation for its spinlock but rather
> its own assembler locking code. This means the writes aren't
> instrumented by the compiler, while some reads are. (see
> __arch_spin_trylock in e.g. arch/powerpc/include/asm/spinlock.h)
>
> Would the correct way to deal with this be for the powerpc code to call
> out to __tsan_readN/__tsan_writeN before invoking the assembler that
> reads and writes the lock?

This should not be the issue, because with KCSAN, not instrumenting
something does not lead to false positives. If two accesses are
involved in a race, and neither of them are instrumented, KCSAN will
not report a race; if however, 1 of them is instrumented (and the
uninstrumented access is a write), KCSAN will infer a race due to the
data value changed ("race at unknown origin").

Rather, if there is spinlock code causing data-races, then there are 2 options:
1) Actually missing READ_ONCE/WRITE_ONCE somewhere.
2) You need to disable instrumentation for an entire function with
__no_sanitize_thread or __no_kcsan_or_inline (for inline functions).
This should only be needed for arch-specific code (e.g. see the
changes we made to arch/x86).

Note: you can explicitly add instrumentation to uninstrumented
accesses with the API in <linux/kcsan-checks.h>, but this shouldn't be
the issue here.

It would be good to symbolize the stack-traces, as otherwise it's hard
to say exactly what needs to be done.

Best,
-- Marco

> Regards,
> Daniel
>
>
> [   24.612864] ==================================================================
> [   24.614188] BUG: KCSAN: racing read in __spin_yield+0xa8/0x180
> [   24.614669]
> [   24.614799] race at unknown origin, with read to 0xc00000003fff9d00 of 4 bytes by task 449 on cpu 11:
> [   24.616024]  __spin_yield+0xa8/0x180
> [   24.616377]  _raw_spin_lock_irqsave+0x1a8/0x1b0
> [   24.616850]  release_pages+0x3a0/0x880
> [   24.617203]  free_pages_and_swap_cache+0x13c/0x220
> [   24.622548]  tlb_flush_mmu+0x210/0x2f0
> [   24.622979]  tlb_finish_mmu+0x12c/0x240
> [   24.623286]  exit_mmap+0x138/0x2c0
> [   24.623779]  mmput+0xe0/0x330
> [   24.624504]  do_exit+0x65c/0x1050
> [   24.624835]  do_group_exit+0xb4/0x210
> [   24.625458]  __wake_up_parent+0x0/0x80
> [   24.625985]  system_call+0x5c/0x70
> [   24.626415]
> [   24.626651] Reported by Kernel Concurrency Sanitizer on:
> [   24.628329] CPU: 11 PID: 449 Comm: systemd-bless-b Not tainted 5.3.0-00007-gad29ff6c190d-dirty #9
> [   24.629508] ==================================================================
>
> [   24.672860] ==================================================================
> [   24.675901] BUG: KCSAN: data-race in _raw_spin_lock_irqsave+0x13c/0x1b0 and _raw_spin_unlock_irqrestore+0x94/0x100
> [   24.680847]
> [   24.682743] write to 0xc0000001ffeefe00 of 4 bytes by task 455 on cpu 5:
> [   24.683402]  _raw_spin_unlock_irqrestore+0x94/0x100
> [   24.684593]  release_pages+0x250/0x880
> [   24.685148]  free_pages_and_swap_cache+0x13c/0x220
> [   24.686068]  tlb_flush_mmu+0x210/0x2f0
> [   24.690190]  tlb_finish_mmu+0x12c/0x240
> [   24.691082]  exit_mmap+0x138/0x2c0
> [   24.693216]  mmput+0xe0/0x330
> [   24.693597]  do_exit+0x65c/0x1050
> [   24.694170]  do_group_exit+0xb4/0x210
> [   24.694658]  __wake_up_parent+0x0/0x80
> [   24.696230]  system_call+0x5c/0x70
> [   24.700414]
> [   24.712991] read to 0xc0000001ffeefe00 of 4 bytes by task 454 on cpu 20:
> [   24.714419]  _raw_spin_lock_irqsave+0x13c/0x1b0
> [   24.715018]  pagevec_lru_move_fn+0xfc/0x1d0
> [   24.715527]  __lru_cache_add+0x124/0x1a0
> [   24.716072]  lru_cache_add+0x30/0x50
> [   24.716411]  add_to_page_cache_lru+0x134/0x250
> [   24.717938]  mpage_readpages+0x220/0x3f0
> [   24.719737]  blkdev_readpages+0x50/0x80
> [   24.721891]  read_pages+0xb4/0x340
> [   24.722834]  __do_page_cache_readahead+0x318/0x350
> [   24.723290]  force_page_cache_readahead+0x150/0x280
> [   24.724391]  page_cache_sync_readahead+0xe4/0x110
> [   24.725087]  generic_file_buffered_read+0xa20/0xdf0
> [   24.727003]  generic_file_read_iter+0x220/0x310
> [   24.728906]
> [   24.730044] Reported by Kernel Concurrency Sanitizer on:
> [   24.732185] CPU: 20 PID: 454 Comm: systemd-gpt-aut Not tainted 5.3.0-00007-gad29ff6c190d-dirty #9
> [   24.734317] ==================================================================
>
>
> >
> > Thanks,
> > -- Marco
>
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