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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