From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from userp2130.oracle.com ([156.151.31.86]:45780 "EHLO userp2130.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161302AbeE1Okz (ORCPT ); Mon, 28 May 2018 10:40:55 -0400 Received: from pps.filterd (userp2130.oracle.com [127.0.0.1]) by userp2130.oracle.com (8.16.0.22/8.16.0.22) with SMTP id w4SEesTG190363 for ; Mon, 28 May 2018 14:40:54 GMT Received: from aserv0021.oracle.com (aserv0021.oracle.com [141.146.126.233]) by userp2130.oracle.com with ESMTP id 2j6y185ksa-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK) for ; Mon, 28 May 2018 14:40:54 +0000 Received: from userv0121.oracle.com (userv0121.oracle.com [156.151.31.72]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id w4SEerBV025327 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK) for ; Mon, 28 May 2018 14:40:53 GMT Received: from abhmp0005.oracle.com (abhmp0005.oracle.com [141.146.116.11]) by userv0121.oracle.com (8.14.4/8.13.8) with ESMTP id w4SEeruu009275 for ; Mon, 28 May 2018 14:40:53 GMT From: Anand Jain To: linux-btrfs@vger.kernel.org Subject: [PATCH 2nd try v4] btrfs: drop uuid_mutex in btrfs_free_extra_devids() Date: Mon, 28 May 2018 22:43:29 +0800 Message-Id: <20180528144329.7790-1-anand.jain@oracle.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: btrfs_free_extra_devids() is called only in the mount context which traverses through the fs_devices::devices and frees the orphan devices devices in the given %fs_devices if any. As the search for the orphan device is limited to fs_devices::devices so we don't need the global uuid_mutex. There can't be any mount-point based ioctl threads in this context as the mount thread is not yet returned. But there can be the btrfs-control based scan ioctls thread which calls device_list_add(). Here in the mount thread the fs_devices::opened is incremented way before btrfs_free_extra_devids() is called and in the scan context the fs_devices which are already opened neither be freed or alloc-able at device_list_add(). But lets say you change the device-path and call the scan again, then scan would update the new device path and this operation could race against the btrfs_free_extra_devids() thread, which might be in the process of free-ing the same device. So synchronize it by using the device_list_mutex. This scenario is a very corner case, and practically the scan and mount are anyway serialized by the usage so unless the race is instrumented its very difficult to achieve. Signed-off-by: Anand Jain --- (I didn't see this email in the mailing list, so trying again). v3->v4: As we traverse through the seed device, fs_device gets updated with the child seed fs_devices, so make sure we use the same fs_devices pointer for the mutex_unlock as used for the mutex_lock. v2->v3: Update change log. (Currently device_list_add() is very lean on its device_list_mutex usage, a cleanup and fix is wip. Given the practicality of the above race condition this patch is good to merge). v1->v2: replace uuid_mutex with device_list_mutex instead of delete. change log updated. fs/btrfs/volumes.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index b6757b53c297..f03719221fca 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -924,8 +924,9 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step) { struct btrfs_device *device, *next; struct btrfs_device *latest_dev = NULL; + struct btrfs_fs_devices *parent_fs_devices = fs_devices; - mutex_lock(&uuid_mutex); + mutex_lock(&parent_fs_devices->device_list_mutex); again: /* This is the initialized path, it is safe to release the devices. */ list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) { @@ -979,8 +980,7 @@ void btrfs_free_extra_devids(struct btrfs_fs_devices *fs_devices, int step) } fs_devices->latest_bdev = latest_dev->bdev; - - mutex_unlock(&uuid_mutex); + mutex_unlock(&parent_fs_devices->device_list_mutex); } static void free_device_rcu(struct rcu_head *head) -- 2.7.0