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Re: [PATCH] tell gcc optimizer to never introduce new data races

On Tue, 10 Jun 2014, Linus Torvalds wrote:

> > We have been chasing a memory corruption bug, which turned out to be
> > caused by very old gcc (4.3.4), which happily turned conditional load into
> > a non-conditional one, and that broke correctness (the condition was met
> > only if lock was held) and corrupted memory.
> Just out of interest, can you point to the particular kernel code that
> caused this? I think that's more interesting than the example program
> you show - which I'm sure is really nice for gcc developers as an
> example, but from a kernel standpoint I think it's more important to
> show the particular problems this caused for the kernel?

Well, as I said, that was with gcc 4.3.4, and GCC people expressed 
themselves that that was a slightly different optimization. It made me 
nervous enough though to ask whether it's absolutely positively not going 
to happen with newer gccs, and the code snippet quoted in the original 
mail came back as a response.

The code in question was out-of-tree printk-in-NMI (yeah, surprise 
suprise, once again) patch written by Petr Mladek, let me quote his 
comment from our internal bugzilla:

I have spent few days investigating inconsistent state of kernel ring buffer.
It went out that it was caused by speculative store generated by 

The problem is in assembly generated for make_free_space(). The functions is
called the following way:

+ vprintk_emit();
    + log = MAIN_LOG; // with logbuf_lock
       log = NMI_LOG; // with nmi_logbuf_lock
       cont_add(log, ...);
        + cont_flush(log, ...);
            + log_store(log, ...);
                  + log_make_free_space(log, ...);

If called with log = NMI_LOG then only nmi_log_* global variables are safe to
modify but the generated code does store also into (main_)log_* global 

       55                      push   %rbp
       89 f6                   mov    %esi,%esi

       48 8b 05 03 99 51 01    mov    0x1519903(%rip),%rax       # ffffffff82620868 <nmi_log_next_id>
       44 8b 1d ec 98 51 01    mov    0x15198ec(%rip),%r11d      # ffffffff82620858 <log_next_idx>
       8b 35 36 60 14 01       mov    0x1146036(%rip),%esi       # ffffffff8224cfa8 <log_buf_len>
       44 8b 35 33 60 14 01    mov    0x1146033(%rip),%r14d      # ffffffff8224cfac <nmi_log_buf_len>
       4c 8b 2d d0 98 51 01    mov    0x15198d0(%rip),%r13       # ffffffff82620850 <log_next_seq>
       4c 8b 25 11 61 14 01    mov    0x1146111(%rip),%r12       # ffffffff8224d098 <log_buf>
       49 89 c2                mov    %rax,%r10
       48 21 c2                and    %rax,%rdx
       48 8b 1d 0c 99 55 01    mov    0x155990c(%rip),%rbx       # ffffffff826608a0 <nmi_log_buf>
       49 c1 ea 20             shr    $0x20,%r10
       48 89 55 d0             mov    %rdx,-0x30(%rbp)
       44 29 de                sub    %r11d,%esi
       45 29 d6                sub    %r10d,%r14d
       4c 8b 0d 97 98 51 01    mov    0x1519897(%rip),%r9	# ffffffff82620840 <log_first_seq>
       eb 7e                   jmp    ffffffff81107029	<log_make_free_space+0xe9>
       85 ff                   test   %edi,%edi                  # edi = 1 for NMI_LOG
       4c 89 e8                mov    %r13,%rax
       4c 89 ca                mov    %r9,%rdx
       74 0a                   je     ffffffff8110703d	<log_make_free_space+0xfd>
       8b 15 27 98 51 01       mov    0x1519827(%rip),%edx       # ffffffff82620860 <nmi_log_first_id>
       48 8b 45 d0             mov    -0x30(%rbp),%rax
       48 39 c2                cmp    %rax,%rdx                  # end of loop 
       0f 84 da 00 00 00       je     ffffffff81107120 <log_make_free_space+0x1e0>
       85 ff                   test   %edi,%edi                  # edi = 1 for NMI_LOG
       4c 89 0d 17 97 51 01    mov    %r9,0x1519717(%rip)        # ffffffff82620840 <log_first_seq>
       74 35                   je     ffffffff81107160		 <log_make_free_space+0x220>

It stores log_first_seq when edi == NMI_LOG. This instructions are used also
when edi == MAIN_LOG but the store is done speculatively before the condition
is decided.  It is unsafe because we do not have "logbuf_lock" in NMI context
and some other process migh modify "log_first_seq" in parallel.

I believe that the best course of action is both

- building kernel (and anything multi-threaded, I guess) with that 
  optimization turned off
- persuade gcc folks to change the default for future releases


Jiri Kosina

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