__cache_alloc

[Borislav Petkov <petkov@xxxxxxxxxxxxxxx>]

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Hi there again,

I've been trying to understand what's going on here but I'm not sure:

<mm/slab.c>
2047 static inline void * __cache_alloc (kmem_cache_t *cachep, int flags)
2048 {
2049         unsigned long save_flags;
2050         void* objp;
2051         struct array_cache *ac;
2052 
2053         cache_alloc_debugcheck_before(cachep, flags);
2054 
2055         local_irq_save(save_flags);
2056         ac = ac_data(cachep);
2057         if (likely(ac->avail)) {
2058                 STATS_INC_ALLOCHIT(cachep);
2059                 ac->touched = 1;
2060                 objp = ac_entry(ac)[--ac->avail];
2061         } else {
2062                 STATS_INC_ALLOCMISS(cachep);
2063                 objp = cache_alloc_refill(cachep, flags);
2064         }
2065         local_irq_restore(save_flags);
2066         objp = cache_alloc_debugcheck_after(cachep, flags, objp, 
__builtin_return_address(0));
2067         return objp;
2068 }

What puzzles me is line 2060: ac_entry(ac) returns (void **)(ac+1), which, if 
I see it correctly, returns a pointer to the next array_cache struct in the 
per CPU *array in the kmem_cache_t struct. But what exactly do the array 
subscript brackets after it do and an object pointer of what kind gets 
written in objp? --ac->avail decrements the count of available slab objects 
in the cache but (ac+1)[--ac->avail] returns the ac->touched of the previous 
array_cache struct which has just been set to 1 in the previous line. hmm, 
mighty strange ?!

Thanks in advance,
Boris.
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