From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id 556A96B03D3 for ; Fri, 23 Jun 2017 06:34:27 -0400 (EDT) Received: by mail-wr0-f199.google.com with SMTP id z81so11609481wrc.2 for ; Fri, 23 Jun 2017 03:34:27 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id m65si3755207wmh.82.2017.06.23.03.34.25 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 23 Jun 2017 03:34:25 -0700 (PDT) Date: Fri, 23 Jun 2017 12:34:23 +0200 From: Michal Hocko Subject: Re: [RFC PATCH] mm, swap: don't disable preemption while taking the per-CPU cache Message-ID: <20170623103423.GJ5308@dhcp22.suse.cz> References: <20170623101254.k4zzbf3dfoukoxkq@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170623101254.k4zzbf3dfoukoxkq@linutronix.de> Sender: owner-linux-mm@kvack.org List-ID: To: Sebastian Andrzej Siewior Cc: Tim Chen , tglx@linutronix.de, ying.huang@intel.com, linux-mm@kvack.org On Fri 23-06-17 12:12:54, Sebastian Andrzej Siewior wrote: > get_cpu_var() disables preemption and returns the per-CPU version of the > variable. Disabling preemption is useful to ensure atomic access to the > variable within the critical section. > In this case however, after the per-CPU version of the variable is > obtained the ->free_lock is acquired. For that reason it seems the raw > accessor could be used. It only seems that ->slots_ret should be > retested (because with disabled preemption this variable can not be set > to NULL otherwise). The changelog doesn't explain, why does this change matter. Disabling preemption shortly before taking a spinlock shouldn't make much difference. I suspect you care because of RT, right? In that case spell that in the changelog and explain why it matters. Other than hat the patch looks good to me. > Signed-off-by: Sebastian Andrzej Siewior > --- > mm/swap_slots.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/mm/swap_slots.c b/mm/swap_slots.c > index 58f6c78f1dad..51c304477482 100644 > --- a/mm/swap_slots.c > +++ b/mm/swap_slots.c > @@ -272,11 +272,11 @@ int free_swap_slot(swp_entry_t entry) > { > struct swap_slots_cache *cache; > > - cache = &get_cpu_var(swp_slots); > + cache = raw_cpu_ptr(&swp_slots); > if (use_swap_slot_cache && cache->slots_ret) { > spin_lock_irq(&cache->free_lock); > /* Swap slots cache may be deactivated before acquiring lock */ > - if (!use_swap_slot_cache) { > + if (!use_swap_slot_cache || !cache->slots_ret) { > spin_unlock_irq(&cache->free_lock); > goto direct_free; > } > @@ -296,7 +296,6 @@ int free_swap_slot(swp_entry_t entry) > direct_free: > swapcache_free_entries(&entry, 1); > } > - put_cpu_var(swp_slots); > > return 0; > } > -- > 2.13.1 > -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org