From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id C3D8220D77DCB for ; Thu, 6 Apr 2017 14:29:55 -0700 (PDT) From: Ross Zwisler Subject: [PATCH] dax: fix radix tree insertion race Date: Thu, 6 Apr 2017 15:29:44 -0600 Message-Id: <20170406212944.2866-1-ross.zwisler@linux.intel.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-nvdimm-bounces@lists.01.org Sender: "Linux-nvdimm" To: Andrew Morton , linux-kernel@vger.kernel.org Cc: Jan Kara , "Darrick J. Wong" , Matthew Wilcox , linux-nvdimm@lists.01.org, Alexander Viro , linux-fsdevel@vger.kernel.org, Christoph Hellwig List-ID: While running generic/340 in my test setup I hit the following race. It can happen with kernels that support FS DAX PMDs, so v4.10 thru v4.11-rc5. Thread 1 Thread 2 -------- -------- dax_iomap_pmd_fault() grab_mapping_entry() spin_lock_irq() get_unlocked_mapping_entry() 'entry' is NULL, can't call lock_slot() spin_unlock_irq() radix_tree_preload() dax_iomap_pmd_fault() grab_mapping_entry() spin_lock_irq() get_unlocked_mapping_entry() ... lock_slot() spin_unlock_irq() dax_pmd_insert_mapping() spin_lock_irq() __radix_tree_insert() fails with -EEXIST The issue is that we have to drop mapping->tree_lock while calling radix_tree_preload(), but since we didn't have a radix tree entry to lock (unlike in the pmd_downgrade case) we have no protection against Thread 2 coming along and inserting a PMD at the same index. For 4k entries we handled this with a special-case response to -EEXIST coming from the __radix_tree_insert(), but this doesn't save us for PMDs because the -EEXIST case can also mean that we collided with a 4k entry in the radix tree at a different index, but one that is covered by our PMD range. So, correctly handle both the 4k and 2M collision cases by explicitly re-checking the radix tree for an entry at our index once we reacquire mapping->tree_lock. This patch has made it through a clean xfstests run with the current v4.11-rc5 based linux/master, and it also ran generic/340 500 times in a loop. It used to fail within the first 10 iterations. Signed-off-by: Ross Zwisler Cc: [4.10+] --- fs/dax.c | 35 ++++++++++++++++++++++------------- 1 file changed, 22 insertions(+), 13 deletions(-) diff --git a/fs/dax.c b/fs/dax.c index de622d4..85abd74 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -373,6 +373,22 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index, } spin_lock_irq(&mapping->tree_lock); + if (!entry) { + /* + * We needed to drop the page_tree lock while calling + * radix_tree_preload() and we didn't have an entry to + * lock. See if another thread inserted an entry at + * our index during this time. + */ + entry = __radix_tree_lookup(&mapping->page_tree, index, + NULL, &slot); + if (entry) { + radix_tree_preload_end(); + spin_unlock_irq(&mapping->tree_lock); + goto restart; + } + } + if (pmd_downgrade) { radix_tree_delete(&mapping->page_tree, index); mapping->nrexceptional--; @@ -388,19 +404,12 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index, if (err) { spin_unlock_irq(&mapping->tree_lock); /* - * Someone already created the entry? This is a - * normal failure when inserting PMDs in a range - * that already contains PTEs. In that case we want - * to return -EEXIST immediately. - */ - if (err == -EEXIST && !(size_flag & RADIX_DAX_PMD)) - goto restart; - /* - * Our insertion of a DAX PMD entry failed, most - * likely because it collided with a PTE sized entry - * at a different index in the PMD range. We haven't - * inserted anything into the radix tree and have no - * waiters to wake. + * Our insertion of a DAX entry failed, most likely + * because we were inserting a PMD entry and it + * collided with a PTE sized entry at a different + * index in the PMD range. We haven't inserted + * anything into the radix tree and have no waiters to + * wake. */ return ERR_PTR(err); } -- 2.9.3 _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm