From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yw0-f199.google.com (mail-yw0-f199.google.com [209.85.161.199]) by kanga.kvack.org (Postfix) with ESMTP id 14B436B0007 for ; Tue, 6 Mar 2018 12:34:56 -0500 (EST) Received: by mail-yw0-f199.google.com with SMTP id q40so1701719ywa.8 for ; Tue, 06 Mar 2018 09:34:56 -0800 (PST) From: Tejun Heo Subject: [PATCH 4/7] HMM: Remove superflous RCU protection around radix tree lookup Date: Tue, 6 Mar 2018 09:33:13 -0800 Message-Id: <20180306173316.3088458-4-tj@kernel.org> In-Reply-To: <20180306173316.3088458-1-tj@kernel.org> References: <20180306172657.3060270-1-tj@kernel.org> <20180306173316.3088458-1-tj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: torvalds@linux-foundation.org, jannh@google.com, paulmck@linux.vnet.ibm.com, bcrl@kvack.org, viro@zeniv.linux.org.uk, kent.overstreet@gmail.com Cc: security@kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Tejun Heo , =?UTF-8?q?J=C3=A9r=C3=B4me=20Glisse?= , linux-mm@kvack.org 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 Cc: JA(C)rA'me Glisse Cc: linux-mm@kvack.org Cc: Linus Torvalds --- Hello, JA(C)rA'me. 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. 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/ . 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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 Cc: Jérôme Glisse Cc: linux-mm@kvack.org Cc: Linus Torvalds --- Hello, Jérôme. 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. 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