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mm, slub: simplify kmem_cache_cpu and tid setup

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In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee that
our kmem_cache_cpu pointer is from the same cpu as the tid value. Currently
that's done by reading the tid first using this_cpu_read(), then the
kmem_cache_cpu pointer and verifying we read the same tid using the pointer and
plain READ_ONCE().

This can be simplified to just fetching kmem_cache_cpu pointer and then reading
tid using the pointer. That guarantees they are from the same cpu. We don't
need to read the tid using this_cpu_read() because the value will be validated
by this_cpu_cmpxchg_double(), making sure we are on the correct cpu and the
freelist didn't change by anyone preempting us since reading the tid.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
This commit is contained in:
Vlastimil Babka 2021-05-18 02:01:39 +02:00
parent 1572df7cbc
commit 9b4bc85a69
1 changed files with 9 additions and 13 deletions
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22
mm/slub.c
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@ -2882,15 +2882,14 @@ redo:
* reading from one cpu area. That does not matter as long
* as we end up on the original cpu again when doing the cmpxchg.
*
* We should guarantee that tid and kmem_cache are retrieved on
* the same cpu. It could be different if CONFIG_PREEMPTION so we need
* to check if it is matched or not.
* We must guarantee that tid and kmem_cache_cpu are retrieved on the
* same cpu. We read first the kmem_cache_cpu pointer and use it to read
* the tid. If we are preempted and switched to another cpu between the
* two reads, it's OK as the two are still associated with the same cpu
* and cmpxchg later will validate the cpu.
*/
do {
tid = this_cpu_read(s->cpu_slab->tid);
c = raw_cpu_ptr(s->cpu_slab);
} while (IS_ENABLED(CONFIG_PREEMPTION) &&
unlikely(tid != READ_ONCE(c->tid)));
c = raw_cpu_ptr(s->cpu_slab);
tid = READ_ONCE(c->tid);
/*
* Irqless object alloc/free algorithm used here depends on sequence
@ -3164,11 +3163,8 @@ redo:
* data is retrieved via this pointer. If we are on the same cpu
* during the cmpxchg then the free will succeed.
*/
do {
tid = this_cpu_read(s->cpu_slab->tid);
c = raw_cpu_ptr(s->cpu_slab);
} while (IS_ENABLED(CONFIG_PREEMPTION) &&
unlikely(tid != READ_ONCE(c->tid)));
c = raw_cpu_ptr(s->cpu_slab);
tid = READ_ONCE(c->tid);
/* Same with comment on barrier() in slab_alloc_node() */
barrier();