Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756952Ab3H2T2X (ORCPT ); Thu, 29 Aug 2013 15:28:23 -0400 Received: from usindpps05.hds.com ([207.126.252.18]:37326 "EHLO usindpps05.hds.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756921Ab3H2T2V convert rfc822-to-8bit (ORCPT ); Thu, 29 Aug 2013 15:28:21 -0400 From: Tomoki Sekiyama To: Vivek Goyal CC: "linux-kernel@vger.kernel.org" , "axboe@kernel.dk" , Seiji Aguchi , "majianpeng@gmail.com" , Tejun Heo Subject: Re: [PATCH] elevator: Fix a race in elevator switching and md device initialization Thread-Topic: [PATCH] elevator: Fix a race in elevator switching and md device initialization Thread-Index: AQHOomKEe8NHX1i8QUyRMlMbtDjKDpmsyrUA///MRYA= Date: Thu, 29 Aug 2013 19:28:02 +0000 Message-ID: In-Reply-To: <20130829183310.GB6171@redhat.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.74.73.11] Content-Type: text/plain; charset="us-ascii" Content-ID: <40261091BC0035418C409AC099F94753@hds.com> Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 X-Proofpoint-SPF-Result: pass X-Proofpoint-SPF-Record: v=spf1 mx ip4:207.126.244.0/26 ip4:207.126.252.0/25 include:mktomail.com include:cloud.hds.com ~all X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:5.10.8794,1.0.431,0.0.0000 definitions=2013-08-29_07:2013-08-29,2013-08-29,1970-01-01 signatures=0 X-Proofpoint-Spam-Details: rule=notspam policy=outbound_policy score=0 spamscore=0 suspectscore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=7.0.1-1305240000 definitions=main-1308290106 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4493 Lines: 127 Hi vivek, Thanks for your comments. On 8/29/13 14:33 , "Vivek Goyal" wrote: >On Mon, Aug 26, 2013 at 09:45:15AM -0400, Tomoki Sekiyama wrote: >> The soft lockup below happes at the boot time of the system using dm >> multipath and automated elevator switching udev rules. >> >> [ 356.127001] BUG: soft lockup - CPU#3 stuck for 22s! [sh:483] >> [ 356.127001] RIP: 0010:[] [] >>lock_timer_base.isra.35+0x1d/0x50 >> ... >> [ 356.127001] Call Trace: >> [ 356.127001] [] try_to_del_timer_sync+0x20/0x70 >> [ 356.127001] [] ? >>kmem_cache_alloc_node_trace+0x20a/0x230 >> [ 356.127001] [] del_timer_sync+0x52/0x60 >> [ 356.127001] [] cfq_exit_queue+0x32/0xf0 >> [ 356.127001] [] elevator_exit+0x2f/0x50 >> [ 356.127001] [] elevator_change+0xf1/0x1c0 >> [ 356.127001] [] elv_iosched_store+0x20/0x50 >> [ 356.127001] [] queue_attr_store+0x59/0xb0 >> [ 356.127001] [] sysfs_write_file+0xc6/0x140 >> [ 356.127001] [] vfs_write+0xbd/0x1e0 >> [ 356.127001] [] SyS_write+0x49/0xa0 >> [ 356.127001] [] system_call_fastpath+0x16/0x1b >> > >Tokomi, > >As you noticed, there is a fedora bug open with similar signature. May >be this patch will fix that issue also. > >https://bugzilla.redhat.com/show_bug.cgi?id=902012 > > >> This is caused by a race between md device initialization and sysfs knob >> to switch the scheduler. >> >> * multipathd: >> SyS_ioctl -> do_vfs_ioctl -> dm_ctl_ioctl -> ctl_ioctl -> table_load >> -> dm_setup_md_queue -> blk_init_allocated_queue -> elevator_init: >> >> q->elevator = elevator_alloc(q, e); // not yet initialized >> >> >>* sh -c 'echo deadline > /sys/$DEVPATH/queue/scheduler' >> SyS_write -> vfs_write -> sysfs_write_file -> queue_attr_store >> ( mutex_lock(&q->sysfs_lock) here. ) >> -> elv_iosched_store -> elevator_change: >> >> >> elevator_exit(old); // try to de-init uninitialized elevator and hang >>up >> >> >>This patch adds acquisition of q->sysfs_lock in >>blk_init_allocated_queue(). >> This also adds the lock into elevator_change() to ensure locking from >>the >> other path, as it is exposed function (and queue_attr_store will uses >> __elevator_change() now, the non-locking version of elevator_change()). > >I think introducing __elevator_change() is orthogonal to this problem. >May be keep that in a separate patch. OK, I will split it into 2 patches. >> block/blk-core.c | 6 +++++- >> block/elevator.c | 16 ++++++++++++++-- >> 2 files changed, 19 insertions(+), 3 deletions(-) >> >> diff --git a/block/blk-core.c b/block/blk-core.c >> index 93a18d1..2323ec3 100644 >> --- a/block/blk-core.c >> +++ b/block/blk-core.c >> @@ -739,9 +739,13 @@ blk_init_allocated_queue(struct request_queue *q, >>request_fn_proc *rfn, >> >> q->sg_reserved_size = INT_MAX; >> >> + /* Protect q->elevator from elevator_change */ >> + mutex_lock(&q->sysfs_lock); >> /* init elevator */ >> if (elevator_init(q, NULL)) >> - return NULL; >> + q = NULL; >> + mutex_unlock(&q->sysfs_lock); >> + > >So core of the problem is, what's the locking semantics to make sure >that we are not trying to switch elevator while it is still initializing. >IOW, should we allow multiple parallel calls of elevator_init_fn() on a >queue and is it safe? > >I would argue that it is easier to read and maintain the code if we >provide explicit locking around. So I like the idea of introducing >some locking around elevator_init(). > >Because we are racing against elevator switch path which takes >q->sysfs_lock, it makes sense to provide mutual exlusion using >q->sysfs_lock. > >What I don't know is that can we take mutex in queue init path. Generally >drivers call it and do they expect that they can call this function >while holding a spin lock. As elevator_alloc() allocates memory with GFP_KERNEL, elevator_init() might sleep. So it should be safe to use mutex here. >I am CCing Tejun also to the thread. He also might have some ideas here. > >Thanks >Vivek Thanks, Tomoki Sekiyama -- 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/