linux-kernel - Re: [PATCH-v2 2/4] target: Drop lun_sep_lock for se_lun->lun_se

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Date:	Tue, 26 May 2015 22:29:45 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	"Nicholas A. Bellinger" <nab@...erainc.com>,
	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	Sagi Grimberg <sagig@...lanox.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH-v2 2/4] target: Drop lun_sep_lock for se_lun->lun_se_dev
 RCU usage

On Tue, 2015-05-26 at 16:30 +0200, Bart Van Assche wrote:
> On 05/26/15 08:57, Nicholas A. Bellinger wrote:
> > @@ -625,6 +626,7 @@ int core_dev_add_initiator_node_lun_acl(
> >   	u32 lun_access)
> >   {
> >   	struct se_node_acl *nacl = lacl->se_lun_nacl;
> > +	struct se_device *dev = lockless_dereference(lun->lun_se_dev);
> >   
> >   	if (!nacl)
> >   		return -EINVAL;
> 
> An attempt to run this code on a system with RCU debugging enabled
> resulted in the following complaint:
> 
> ===============================
> [ INFO: suspicious RCU usage. ]
> 4.1.0-rc1-lio-dbg+ #1 Not tainted
> -------------------------------
> drivers/target/target_core_device.c:617 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 1
> 2 locks held by ln/1497:
>  #0:  (sb_writers#11){.+.+.+}, at: [<ffffffff811d9ca4>] mnt_want_write+0x24/0x50
>  #1:  (&sb->s_type->i_mutex_key#14/1){+.+.+.}, at: [<ffffffff811c4cdd>] filename_create+0xad/0x1a0
> 
> stack backtrace:
> CPU: 0 PID: 1497 Comm: ln Not tainted 4.1.0-rc1-lio-dbg+ #1
> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
>  0000000000000001 ffff88005955bd68 ffffffff814fa346 0000000000000011
>  ffff880058bf1270 ffff88005955bd98 ffffffff810ab235 ffff880050db9a68
>  ffff880058ae2e68 0000000000000002 ffff880058ae4120 ffff88005955be08
> Call Trace:
>  [<ffffffff814fa346>] dump_stack+0x4f/0x7b
>  [<ffffffff810ab235>] lockdep_rcu_suspicious+0xd5/0x110
>  [<ffffffffa04324bc>] core_dev_add_initiator_node_lun_acl+0xec/0x190 [target_core_mod]
>  [<ffffffff8108f871>] ? get_parent_ip+0x11/0x50
>  [<ffffffffa04346f9>] target_fabric_mappedlun_link+0x129/0x240 [target_core_mod]
>  [<ffffffffa043466c>] ? target_fabric_mappedlun_link+0x9c/0x240 [target_core_mod]
>  [<ffffffffa035824d>] configfs_symlink+0x13d/0x360 [configfs]
>  [<ffffffff811be8c8>] vfs_symlink+0x58/0xb0
>  [<ffffffff811c75c5>] SyS_symlink+0x65/0xc0
>  [<ffffffff81502eb2>] system_call_fastpath+0x16/0x7a
> 

In this particular case, the se_device behind se_lun->lun_se_dev
__rcu protected pointer can't be released without first releasing the
pre-existing se_lun->lun_group reference to se_device->dev_group.

And since se_lun->lun_group is the source of a configfs symlink to
se_lun_acl->se_lun_group here, the se_lun associated RCU pointer and
underlying se_device can't be released out from under the above
target_fabric_mappedlun_link() code accessing a __rcu protected pointer.

Paul, is lockless_dereference the correct notation for this type of
use-case..?

Thank you,

--nab

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