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SubjectRe: [PATCHv2] mm/slub: fix lockups on PREEMPT && !SMP kernels
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Mark Rutland <mark.rutland <at> arm.com> writes:

>
> Commit 9aabf810a67cd97e ("mm/slub: optimize alloc/free fastpath by
> removing preemption on/off") introduced an occasional hang for kernels
> built with CONFIG_PREEMPT && !CONFIG_SMP.
>
> The problem is the following loop the patch introduced to
> slab_alloc_node and slab_free:
>
> do {
> tid = this_cpu_read(s->cpu_slab->tid);
> c = raw_cpu_ptr(s->cpu_slab);
> } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
>
> GCC 4.9 has been observed to hoist the load of c and c->tid above the
> loop for !SMP kernels (as in this case raw_cpu_ptr(x) is compile-time
> constant and does not force a reload). On arm64 the generated assembly
> looks like:
>
> ffffffc00016d3c4: f9400404 ldr x4, [x0,#8]
> ffffffc00016d3c8: f9400401 ldr x1, [x0,#8]
> ffffffc00016d3cc: eb04003f cmp x1, x4
> ffffffc00016d3d0: 54ffffc1 b.ne ffffffc00016d3c8
<slab_alloc_node.constprop.82+0x30>

Just FYI I've hit this problem on x86. I think I used gcc 4.7 and 4.8, and
maybe also 4.6 earlier.

Here's the diff in asm after applying the fix:

240a: 0f 85 ce 00 00 00 jne 24de <kmem_cache_free+0x10e>
2410: 89 75 f0 mov %esi,-0x10(%ebp)
2413: 8b 07 mov (%edi),%eax
- 2415: 8b 50 04 mov 0x4(%eax),%edx
- 2418: 8b 48 04 mov 0x4(%eax),%ecx
- 241b: 39 d1 cmp %edx,%ecx
- 241d: 75 f9 jne 2418 <kmem_cache_free+0x48>
+ 2415: 8b 48 04 mov 0x4(%eax),%ecx
+ 2418: 8b 50 04 mov 0x4(%eax),%edx
+ 241b: 39 ca cmp %ecx,%edx
+ 241d: 75 f6 jne 2415 <kmem_cache_free+0x45>
241f: 8b 4d f0 mov -0x10(%ebp),%ecx
2422: 39 48 08 cmp %ecx,0x8(%eax)
2425: 0f 85 9a 00 00 00 jne 24c5 <kmem_cache_free+0xf5>

As 3.19+ is broken now, this should go into stable.
Cc: stable@vger.kernel.org



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