Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755635Ab0LQQEE (ORCPT ); Fri, 17 Dec 2010 11:04:04 -0500 Received: from e5.ny.us.ibm.com ([32.97.182.145]:60436 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755339Ab0LQQEC (ORCPT ); Fri, 17 Dec 2010 11:04:02 -0500 Date: Fri, 17 Dec 2010 08:03:29 -0800 From: "Paul E. McKenney" To: Minchan Kim Cc: gerald.schaefer@de.ibm.com, KAMEZAWA Hiroyuki , Milton Miller , linux-kernel@vger.kernel.org, linux-mm@vger.kernel.org, linux-ext4@vger.kernel.org, "Ted Ts'o" , Arun Bhanu , Mel Gorman , Andrew Morton , Heiko Carstens , Martin Schwidefsky Subject: Re: [BUG?] memory hotplug: include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection! Message-ID: <20101217160329.GD2181@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20101121173726.GG23423@thunk.org> <20101122061619.GA2764@linux.vnet.ibm.com> <1291748509.19276.62.camel@thinkpad> <20101208101947.b0646226.kamezawa.hiroyu@jp.fujitsu.com> <1292507421.4885.19.camel@thinkpad> <20101217054722.GG2253@linux.vnet.ibm.com> <20101217150827.GA1609@barrios-desktop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20101217150827.GA1609@barrios-desktop> User-Agent: Mutt/1.5.20 (2009-06-14) X-Content-Scanned: Fidelis XPS MAILER Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7996 Lines: 185 On Sat, Dec 18, 2010 at 12:08:27AM +0900, Minchan Kim wrote: > On Thu, Dec 16, 2010 at 09:47:22PM -0800, Paul E. McKenney wrote: > > On Fri, Dec 17, 2010 at 09:04:13AM +0900, Minchan Kim wrote: > > > On Thu, Dec 16, 2010 at 10:50 PM, Gerald Schaefer > > > wrote: > > > > I got the same warning now after increasing /proc/sys/vm/nr_hugepages, see > > > > below. Both cases are easily reproducible: memory unplug with big page cache, > > > > or adding large pages during run-time. > > > > > > > > =================================================== > > > > [ INFO: suspicious rcu_dereference_check() usage. ] > > > > --------------------------------------------------- > > > > include/linux/radix-tree.h:145 invoked rcu_dereference_check() without protection! > > > > > > > > other info that might help us debug this: > > > > > > > > > > > > rcu_scheduler_active = 1, debug_locks = 0 > > > > 1 lock held by bash/761: > > > > ?#0: ?(&(&inode->i_data.tree_lock)->rlock){..-.-.}, at: [<00000000002263ae>] migrate_page_move_mapping+0x4a/0x2d8 > > > > > > > > stack backtrace: > > > > CPU: 1 Not tainted 2.6.37-rc6 #4 > > > > Process bash (pid: 761, task: 00000000181b5540, ksp: 00000000181bb7f8) > > > > 00000000181bb818 00000000181bb798 0000000000000002 0000000000000000 > > > > ? ? ? 00000000181bb838 00000000181bb7b0 00000000181bb7b0 000000000056bafa > > > > ? ? ? 0000000000000000 000000003f42bdf0 0000000000000002 000000001c43be30 > > > > ? ? ? 000003e00000000d 000003e00000000c 00000000181bb800 0000000000000000 > > > > ? ? ? 0000000000000000 0000000000100bfa 00000000181bb798 00000000181bb7d8 > > > > Call Trace: > > > > ([<0000000000100b02>] show_trace+0xee/0x144) > > > > ?[<000000000022654e>] migrate_page_move_mapping+0x1ea/0x2d8 > > > > ?[<0000000000226c80>] migrate_page+0x38/0x68 > > > > ?[<0000000000226d9a>] move_to_new_page+0xea/0x2bc > > > > ?[<000000000022785a>] migrate_pages+0x496/0x568 > > > > ?[<000000000021e24e>] compact_zone+0x432/0x7d8 > > > > ?[<000000000021e772>] compact_zone_order+0x9e/0xbc > > > > ?[<000000000021ed52>] try_to_compact_pages+0x1ba/0x24c > > > > ?[<00000000001e1afa>] __alloc_pages_nodemask+0x86a/0xa64 > > > > ?[<000000000021c80c>] alloc_fresh_huge_page.clone.2+0x68/0x18c > > > > ?[<000000000021cc4c>] set_max_huge_pages.clone.0+0xa4/0x1ac > > > > ?[<000000000021ce06>] hugetlb_sysctl_handler+0xb2/0xcc > > > > ?[<00000000002a6572>] proc_sys_call_handler+0xe6/0x10c > > > > ?[<00000000002a65be>] proc_sys_write+0x26/0x34 > > > > ?[<00000000002336e0>] vfs_write+0xac/0x18c > > > > ?[<00000000002338bc>] SyS_write+0x58/0xa8 > > > > ?[<0000000000113976>] sysc_noemu+0x16/0x1c > > > > ?[<0000020000162edc>] 0x20000162edc > > > > INFO: lockdep is turned off. > > > > > > > > I honestly do not understand 100% why this is a false positive, seeing that > > > > e.g. find_get_page() will also use radix_tree_deref_slot(), holding only the > > > > rcu_read_lock, while migrate_page_move_mapping() has no rcu_read_lock() but > > > > the &mapping->tree_lock instead. So I'm not quite sure how to fix this > > > > properly, but simply adding rcu_read_lock/unlock() to the affected code paths, > > > > even if it is not necessary for synchronization, would get rid of the warning, > > > > like in the following patch. Any ideas? > > > > > > In case of anon page, we hold rcu_read_lock in unmap_and_move. > > > The problem is file-backed page. In case of that, we hold lock_page > > > and mapping->tree_lock as update-side lock. > > > So we don't need rcu_read_lock. > > > > > > > > > > > --- > > > > ?fs/hugetlbfs/inode.c | ? ?2 ++ > > > > ?mm/migrate.c ? ? ? ? | ? ?4 ++++ > > > > ?2 files changed, 6 insertions(+) > > > > > > > > --- a/fs/hugetlbfs/inode.c > > > > +++ b/fs/hugetlbfs/inode.c > > > > @@ -580,7 +580,9 @@ static int hugetlbfs_migrate_page(struct > > > > ?{ > > > > ? ? ? ?int rc; > > > > > > > > + ? ? ? rcu_read_lock(); > > > > ? ? ? ?rc = migrate_huge_page_move_mapping(mapping, newpage, page); > > > > + ? ? ? rcu_read_unlock(); > > > > ? ? ? ?if (rc) > > > > ? ? ? ? ? ? ? ?return rc; > > > > ? ? ? ?migrate_page_copy(newpage, page); > > > > --- a/mm/migrate.c > > > > +++ b/mm/migrate.c > > > > @@ -417,7 +417,9 @@ int migrate_page(struct address_space *m > > > > > > > > ? ? ? ?BUG_ON(PageWriteback(page)); ? ?/* Writeback must be complete */ > > > > > > > > + ? ? ? rcu_read_lock(); > > > > ? ? ? ?rc = migrate_page_move_mapping(mapping, newpage, page); > > > > + ? ? ? rcu_read_unlock(); > > > > > > > > ? ? ? ?if (rc) > > > > ? ? ? ? ? ? ? ?return rc; > > > > @@ -444,7 +446,9 @@ int buffer_migrate_page(struct address_s > > > > > > > > ? ? ? ?head = page_buffers(page); > > > > > > > > + ? ? ? rcu_read_lock(); > > > > ? ? ? ?rc = migrate_page_move_mapping(mapping, newpage, page); > > > > + ? ? ? rcu_read_unlock(); > > > > > > > > ? ? ? ?if (rc) > > > > ? ? ? ? ? ? ? ?return rc; > > > > > > > > > > > > > > > > > > > > > How about this? > > > Maybe Paul have better idea. > > > (It's apparently be word-wrapped.) > > > > > > diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h > > > index ab2baa5..135af1e 100644 > > > --- a/include/linux/radix-tree.h > > > +++ b/include/linux/radix-tree.h > > > @@ -146,6 +146,20 @@ static inline void *radix_tree_deref_slot(void **pslot) > > > } > > > > > > /** > > > + * radix_tree_deref_slot_nocheck - dereference a slot without RCU check > > > + * @pslot: pointer to slot, returned by radix_tree_lookup_slot > > > + * Returns: item that was stored in that slot with any direct pointer flag > > > + * removed. > > > + * > > > + * This functions works like radix_tree_deref_slot except it doesn't check > > > + * RCU rule. Normally this funcion is used with update-side lock. > > > + * You should use this function very carefully. > > > + */ > > > +static inline void *radix_tree_deref_slot_nocheck(void **pslot) > > > +{ > > > + return rcu_dereference_protected(*pslot, 1); > > > > I suggest replacing the "1" with lockdep expressions for the locks > > that you say might be held: > > It's not hard. > > > > > return rcu_dereference_check(*pslot, > > lockdep_is_held(&mapping->tree_lock)); > > rcu_dereference_check still pass rcu_read_lock_held check we don't want. > I think rcu_dereference_protected is proper. You are exactly right. The only reason that I used rcu_dereference_check() instead of rcu_dereference_protected() is because I didn't realize that RCU readers never called this function. > Why I don't add lockdep expressions is radix_tree_deref_slot is general API. > It might be used anywhere where it doesn't related to mapping->tree_lock. > If we add argument 'mapping', it has a very strong dependency with address_space. > so I decided making the function general and then caller must use it very carefully. > But I am not strong in this point. I believe that this would be a good thing. > > This assumes that when you said "and" you meant both lock_page() and > > mapping->tree_lock. Also you need to pass in the mapping, which > > should not be a problem given likely inlining. > > > > If you meant that either mapping->tree_lock or page_lock() might be > > held, I suppose that the page_lock() state could be passed in, but > > perhaps better to take a general lockdep expression. > > > > So, either or both? ;-) > > > > Thanx, Paul > > I think either is okay. That's because remove_from_page_cache/__remove_from_page_cache > needs both locks so we can't prevent update if we get a either lock. > In code context, I think mapping->tree_lock is more readable since it is used near by. Good!!! So we only really need to check for one or the other. > Thanks for the comment, Paul. No problem! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/