Messages in this thread | | | Date | Wed, 4 May 2011 09:04:19 -0500 (CDT) | From | Christoph Lameter <> | Subject | Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs |
| |
On Wed, 4 May 2011, Pekka Enberg wrote:
> > Well, it's pretty simple: > > > > CONFIG_X86_CMPXCHG64=y compiles this_cpu_generic_cmpxchg_double() into: > > > > 28f7: 89 f9 mov %edi,%ecx > > 28f9: 8b 3e mov (%esi),%edi > > 28fb: 89 45 e4 mov %eax,-0x1c(%ebp) > > 28fe: 89 c3 mov %eax,%ebx > > 2900: 8b 45 f0 mov -0x10(%ebp),%eax > > 2903: 64 0f c7 0f cmpxchg8b %fs:(%edi) > > 2907: 0f 94 c0 sete %al > > 290a: 84 c0 test %al,%al > > 290c: 88 45 e4 mov %al,-0x1c(%ebp) > > 290f: 74 a4 je 28b5 <kmem_cache_alloc+0x29> > > > > while CONFIG_X86_CMPXCHG64=n results in: > > > > 28b0: 8b 03 mov (%ebx),%eax > > 28b2: 64 8b 30 mov %fs:(%eax),%esi > > 28b5: 39 d6 cmp %edx,%esi > > 28b7: 75 d2 jne 288b <kmem_cache_alloc+0x26> > > 28b9: 64 8b 50 04 mov %fs:0x4(%eax),%edx > > 28bd: 39 ca cmp %ecx,%edx > > 28bf: 75 ca jne 288b <kmem_cache_alloc+0x26> > > 28c1: 8b 4b 14 mov 0x14(%ebx),%ecx > > 28c4: 8b 0c 0e mov (%esi,%ecx,1),%ecx > > 28c7: 64 89 08 mov %ecx,%fs:(%eax) > > 28ca: 8b 03 mov (%ebx),%eax > > 28cc: 42 inc %edx > > 28cd: 64 89 50 04 mov %edx,%fs:0x4(%eax) > > > > And that code runs with preemption enabled. So when the task gets > > preempted _BEFORE_ it has actuallty written back the data, then the > > race window is wide open. > > > > I'm still trying to understand that macro hell which actually > > generates that code. I always thought that George Anzingers macro maze > > was horrible, but that's even worse. > > I'm adding Christoph and Tejun to CC.
The above should not happen. If a kernel config indicates that there is no cmpxchg16b/cmpxchg8b available then we need to not compile the path that uses cmpxchg8b/cmpxchg16b. Guess we need CMPXCHG_DOUBLE_LOCAL or so.
| |