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Thu, 5 Aug 2021 15:20:07 +0000 (UTC) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 3AC3A223DC; Thu, 5 Aug 2021 15:20:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1628176806; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Mp7va9uuzrf1g813mTixCnoAulhXUk7Ef+KNQhzo+iI=; b=DdPwE4xCyKxqnAQHcaGImsIjD8YtlmON+ZLUKIBQTH+8Byuyr4rkXQFEaqrx2NqQPlJN5X lQdMLIAyY1POkQhOQatTrjfj+R22zhZMt6OLkZP5DbpPvHdJit9Sl3Zx8i3r+OkMUth/LP y1JutXUA3yk3EYh6YXlKKJOROntPwmc= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1628176806; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Mp7va9uuzrf1g813mTixCnoAulhXUk7Ef+KNQhzo+iI=; b=AVWkjMMMkc0KwJ2qxJXDpV3kRc4XclSR/1mVazFtbIRodzfGGfdFGVTcRHRI0YlEE9B5Ws wM6X5iHvWT6KKEAA== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 0EAB713DAC; Thu, 5 Aug 2021 15:20:06 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id kASWAqYBDGFDdQAAMHmgww (envelope-from ); Thu, 05 Aug 2021 15:20:06 +0000 From: Vlastimil Babka To: Andrew Morton , Christoph Lameter , David Rientjes , Pekka Enberg , Joonsoo Kim Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mike Galbraith , Sebastian Andrzej Siewior , Thomas Gleixner , Mel Gorman , Jesper Dangaard Brouer , Jann Horn , Vlastimil Babka Subject: [PATCH v4 11/35] mm, slub: simplify kmem_cache_cpu and tid setup Date: Thu, 5 Aug 2021 17:19:36 +0200 Message-Id: <20210805152000.12817-12-vbabka@suse.cz> X-Mailer: git-send-email 2.32.0 In-Reply-To: <20210805152000.12817-1-vbabka@suse.cz> References: <20210805152000.12817-1-vbabka@suse.cz> MIME-Version: 1.0 X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 28AEE801CF9E Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=DdPwE4xC; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=AVWkjMMM; dmarc=none; spf=pass (imf06.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.28 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Stat-Signature: f5zf15ccgaygzmh83t6ykuwo96k7i5k4 X-HE-Tag: 1628176807-738924 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: 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 Acked-by: Mel Gorman --- mm/slub.c | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index c32048353645..e2d803b7e3b5 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2865,15 +2865,14 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, * 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 @@ -3147,11 +3146,8 @@ static __always_inline void do_slab_free(struct kmem_cache *s, * 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();