[FFmpeg-devel] [PATCH] avutil: fix data race in av_get_cpu_flags().

[Wan-Teh Chang]

Hi Michael,

On Tue, Nov 22, 2016 at 1:22 PM, Michael Niedermayer
<michael at niedermayer.cc> wrote:
>
> ok, i see th race but do you really need the atomic operations ?
> isnt merging the 2 variables into 1 as you do enough ?
> (i mean the tools might still complain but would there be an actual
>  race remaining?)

The atomic operations avoid the "undefined behavior" resulting from
the data races in the C source code. ThreadSanitizer analyzes the C
source code, therefore it must warn about what may be undefined
behavior according to the C standard, even though for a particular
compiler and processor, the generated code is the same.

Here is a small test program that shows gcc generates the same x86_64
assembly code for the normal and atomic (with relaxed memory ordering)
load and store operations:

==========
$ gcc --version
gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat flags.c
static int flags;

int get_flags_nonatomic(void)
{
    return flags;
}

int get_flags_atomic_relaxed(void)
{
    return __atomic_load_n(&flags, __ATOMIC_RELAXED);
}

void set_flags_nonatomic(int val)
{
    flags = val;
}

void set_flags_atomic_relaxed(int val)
{
    __atomic_store_n(&flags, val, __ATOMIC_RELAXED);
}
$ gcc -Wall -O3 -std=c11 -S -c flags.c
$ cat flags.s
.file "flags.c"
.text
.p2align 4,,15
.globl get_flags_nonatomic
.type get_flags_nonatomic, @function
get_flags_nonatomic:
.LFB0:
.cfi_startproc
movl flags(%rip), %eax
ret
.cfi_endproc
.LFE0:
.size get_flags_nonatomic, .-get_flags_nonatomic
.p2align 4,,15
.globl get_flags_atomic_relaxed
.type get_flags_atomic_relaxed, @function
get_flags_atomic_relaxed:
.LFB1:
.cfi_startproc
movl flags(%rip), %eax
ret
.cfi_endproc
.LFE1:
.size get_flags_atomic_relaxed, .-get_flags_atomic_relaxed
.p2align 4,,15
.globl set_flags_nonatomic
.type set_flags_nonatomic, @function
set_flags_nonatomic:
.LFB2:
.cfi_startproc
movl %edi, flags(%rip)
ret
.cfi_endproc
.LFE2:
.size set_flags_nonatomic, .-set_flags_nonatomic
.p2align 4,,15
.globl set_flags_atomic_relaxed
.type set_flags_atomic_relaxed, @function
set_flags_atomic_relaxed:
.LFB3:
.cfi_startproc
movl %edi, flags(%rip)
ret
.cfi_endproc
.LFE3:
.size set_flags_atomic_relaxed, .-set_flags_atomic_relaxed
.local flags
.comm flags,4,16
.ident "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4"
.section .note.GNU-stack,"", at progbits
==========

Wan-Teh Chang