On Thu, 2 Feb 2012, James Courtier-Dutton wrote: > On 1 February 2012 15:19, Jan Kara <j...@suse.cz> wrote: > > Hello, > > > > 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... > > What is the recommended work around for this problem?
The recommended work around is to re-layout your structures. Richard.