Re: [PATCHv2] mm/slub: fix lockups on PREEMPT && !SMP kernels

[Ville SyrjÃlÃ]

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@xxxxxxxxxxxxxxx

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