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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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