From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f199.google.com (mail-qk0-f199.google.com [209.85.220.199]) by kanga.kvack.org (Postfix) with ESMTP id 225936B0003 for ; Tue, 6 Mar 2018 12:59:52 -0500 (EST) Received: by mail-qk0-f199.google.com with SMTP id n141so1208339qke.20 for ; Tue, 06 Mar 2018 09:59:52 -0800 (PST) Date: Tue, 6 Mar 2018 12:59:48 -0500 From: Jerome Glisse Subject: Re: [PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup Message-ID: <20180306175948.GA4791@redhat.com> References: <20180306172657.3060270-1-tj@kernel.org> <20180306173316.3088458-1-tj@kernel.org> <20180306173316.3088458-4-tj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20180306173316.3088458-4-tj@kernel.org> Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo Cc: torvalds@linux-foundation.org, jannh@google.com, paulmck@linux.vnet.ibm.com, bcrl@kvack.org, viro@zeniv.linux.org.uk, kent.overstreet@gmail.com, security@kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, linux-mm@kvack.org On Tue, Mar 06, 2018 at 09:33:13AM -0800, Tejun Heo wrote: > hmm_devmem_find() requires rcu_read_lock_held() but there's nothing > which actually uses the RCU protection. The only caller is > hmm_devmem_pages_create() which already grabs the mutex and does > superflous rcu_read_lock/unlock() around the function. > > This doesn't add anything and just adds to confusion. Remove the RCU > protection and open-code the radix tree lookup. If this needs to > become more sophisticated in the future, let's add them back when > necessary. > > Signed-off-by: Tejun Heo Reviewed-by: Jerome Glisse > Cc: linux-mm@kvack.org > Cc: Linus Torvalds > --- > Hello, Jerome. > > This came up while auditing percpu_ref users for missing explicit RCU > grace periods. HMM doesn't seem to depend on RCU protection at all, > so I thought it'd be better to remove it for now. It's only compile > tested. Good catch some left over of old logic. I have more cleanup queued up now that i am about to post nouveau patches to use all this. Thanks for fixing this. > > Thanks. > > mm/hmm.c | 12 ++---------- > 1 file changed, 2 insertions(+), 10 deletions(-) > > diff --git a/mm/hmm.c b/mm/hmm.c > index 320545b98..d4627c5 100644 > --- a/mm/hmm.c > +++ b/mm/hmm.c > @@ -845,13 +845,6 @@ static void hmm_devmem_release(struct device *dev, void *data) > hmm_devmem_radix_release(resource); > } > > -static struct hmm_devmem *hmm_devmem_find(resource_size_t phys) > -{ > - WARN_ON_ONCE(!rcu_read_lock_held()); > - > - return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT); > -} > - > static int hmm_devmem_pages_create(struct hmm_devmem *devmem) > { > resource_size_t key, align_start, align_size, align_end; > @@ -892,9 +885,8 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem) > for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) { > struct hmm_devmem *dup; > > - rcu_read_lock(); > - dup = hmm_devmem_find(key); > - rcu_read_unlock(); > + dup = radix_tree_lookup(&hmm_devmem_radix, > + key >> PA_SECTION_SHIFT); > if (dup) { > dev_err(device, "%s: collides with mapping for %s\n", > __func__, dev_name(dup->device)); > -- > 2.9.5 > -- 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