Re: Memory corruption due to word sharing

[Markus Trippelsdorf <markus@trippelsdorf.de>]

On 2012.02.01 at 16:19 +0100, Jan Kara wrote:
>   we've spotted the following mismatch between what kernel folks expect
> from a compiler and what GCC really does, resulting in memory corruption on
> some architectures. Consider the following structure:
> struct x {
>     long a;
>     unsigned int b1;
>     unsigned int b2:1;
> };
> 
> We have two processes P1 and P2 where P1 updates field b1 and P2 updates
> bitfield b2. The code GCC generates for b2 = 1 e.g. on ia64 is:
>    0:   09 00 21 40 00 21       [MMI]       adds r32=8,r32
>    6:   00 00 00 02 00 e0                   nop.m 0x0
>    c:   11 00 00 90                         mov r15=1;;
>   10:   0b 70 00 40 18 10       [MMI]       ld8 r14=[r32];;
>   16:   00 00 00 02 00 c0                   nop.m 0x0
>   1c:   f1 70 c0 47                         dep r14=r15,r14,32,1;;
>   20:   11 00 38 40 98 11       [MIB]       st8 [r32]=r14
>   26:   00 00 00 02 00 80                   nop.i 0x0
>   2c:   08 00 84 00                         br.ret.sptk.many b0;;
> 
> Note that gcc used 64-bit read-modify-write cycle to update b2. Thus if P1
> races with P2, update of b1 can get lost. BTW: I've just checked on x86_64
> and there GCC uses 8-bit bitop to modify the bitfield.
> 
> We actually spotted this race in practice in btrfs on structure
> fs/btrfs/ctree.h:struct btrfs_block_rsv where spinlock content got
> corrupted due to update of following bitfield and there seem to be other
> places in kernel where this could happen.
> 
> I've raised the issue with our GCC guys and they said to me that: "C does
> not provide such guarantee, nor can you reliably lock different
> structure fields with different locks if they share naturally aligned
> word-size memory regions.  The C++11 memory model would guarantee this,
> but that's not implemented nor do you build the kernel with a C++11
> compiler."
> 
> So it seems what C/GCC promises does not quite match with what kernel
> expects. I'm not really an expert in this area so I wanted to report it
> here so that more knowledgeable people can decide how to solve the issue...

FYI, the gcc bug can be found here:
http://gcc.gnu.org/bugzilla/show_bug.cgi?idR080

-- 
Markus