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charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 93923120024 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 78fwic84gxkmks4ayqiaj1yjbqji6q7u X-HE-Tag: 1721672873-275897 X-HE-Meta: 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 JEqaVXaX 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello, On Mon, Jul 22, 2024 at 11:03:00AM -0700, Boqun Feng wrote: > On Mon, Jul 22, 2024 at 07:52:22AM -1000, Tejun Heo wrote: > > On Mon, Jul 22, 2024 at 10:47:30AM -0700, Boqun Feng wrote: > > > This looks like a data race because we read pcpu_nr_empty_pop_pages out > > > of the lock for a best effort checking, @Tejun, maybe you could confirm > > > on this? > > > > That does sound plausible. > > > > > - if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW) > > > + /* > > > + * Checks pcpu_nr_empty_pop_pages out of the pcpu_lock, data races may > > > + * occur but this is just a best-effort checking, everything is synced > > > + * in pcpu_balance_work. > > > + */ > > > + if (data_race(pcpu_nr_empty_pop_pages) < PCPU_EMPTY_POP_PAGES_LOW) > > > pcpu_schedule_balance_work(); > > > > Would it be better to use READ/WRITE_ONCE() for the variable? > > > > For READ/WRITE_ONCE(), we will need to replace all write accesses and > all out-of-lock read accesses to pcpu_nr_empty_pop_pages, like below. > It's better in the sense that it doesn't rely on compiler behaviors on > data races, not sure about the performance impact though. > I think a better alternative is we can move it up into the lock under area_found. The value gets updated as part of pcpu_alloc_area() as the code above populates percpu memory that is already allocated. We should probably annotate pcpu_update_empty_pages() with: lockdep_assert_held(&pcpu_lock); Thanks, Dennis > Regards, > Boqun > > ----->8 > diff --git a/mm/percpu.c b/mm/percpu.c > index 20d91af8c033..729e8188238b 100644 > --- a/mm/percpu.c > +++ b/mm/percpu.c > @@ -570,7 +570,8 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk) > > if (!chunk->isolated) { > chunk->isolated = true; > - pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages; > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > + pcpu_nr_empty_pop_pages - chunk->nr_empty_pop_pages); > } > list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]); > } > @@ -581,7 +582,8 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk) > > if (chunk->isolated) { > chunk->isolated = false; > - pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages; > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > + pcpu_nr_empty_pop_pages + chunk->nr_empty_pop_pages); > pcpu_chunk_relocate(chunk, -1); > } > } > @@ -599,7 +601,8 @@ static inline void pcpu_update_empty_pages(struct pcpu_chunk *chunk, int nr) > { > chunk->nr_empty_pop_pages += nr; > if (chunk != pcpu_reserved_chunk && !chunk->isolated) > - pcpu_nr_empty_pop_pages += nr; > + WRITE_ONCE(pcpu_nr_empty_pop_pages, > + pcpu_nr_empty_pop_pages + nr); > } > > /* > @@ -1891,7 +1894,7 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved, > mutex_unlock(&pcpu_alloc_mutex); > } > > - if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW) > + if (READ_ONCE(pcpu_nr_empty_pop_pages) < PCPU_EMPTY_POP_PAGES_LOW) > pcpu_schedule_balance_work(); > > /* clear the areas and return address relative to base address */ > @@ -2754,7 +2757,7 @@ void __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai, > tmp_addr = (unsigned long)base_addr + static_size + ai->reserved_size; > pcpu_first_chunk = pcpu_alloc_first_chunk(tmp_addr, dyn_size); > > - pcpu_nr_empty_pop_pages = pcpu_first_chunk->nr_empty_pop_pages; > + WRITE_ONCE(pcpu_nr_empty_pop_pages, pcpu_first_chunk->nr_empty_pop_pages); > pcpu_chunk_relocate(pcpu_first_chunk, -1); > > /* include all regions of the first chunk */ >