Re: [PATCH tip/core/rcu 3/9] drivers/md: Use rcu_dereference() for accessing rcu pointer

[Kirill A. Shutemov]

On Tue, Oct 28, 2014 at 03:09:56PM -0700, Paul E. McKenney wrote:
> From: Pranith Kumar <bobby.prani@xxxxxxxxx>
> 
> Got Paul's email wrong the first time.
> 
> The map field in 'struct mapped_device' is an rcu pointer. Use rcu_dereference()
> while accessing it.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

On current -next I see this:

[    6.388264] ===============================
[    6.389571] [ INFO: suspicious RCU usage. ]
[    6.390869] 3.18.0-rc5-next-20141121-08303-g44cae4530372 #2 Not tainted
[    6.392185] -------------------------------
[    6.393479] /home/kas/git/public/linux/drivers/md/dm.c:2853 suspicious rcu_dereference_check() usage!
[    6.394801] 
other info that might help us debug this:

[    6.398714] 
rcu_scheduler_active = 1, debug_locks = 0
[    6.401247] 1 lock held by cryptsetup/159:
[    6.402522]  #0:  (&md->suspend_lock/1){+.+...}, at: [<ffffffff8179dd1d>] dm_suspend+0x3d/0x140
[    6.403848] 
stack backtrace:
[    6.406448] CPU: 3 PID: 159 Comm: cryptsetup Not tainted 3.18.0-rc5-next-20141121-08303-g44cae4530372 #2
[    6.407726] Hardware name: LENOVO 3460CC6/3460CC6, BIOS G6ET93WW (2.53 ) 02/04/2013
[    6.408982]  0000000000000001 ffff8800d3ac7c38 ffffffff81b00bbd 0000000000000011
[    6.410249]  ffff8800d301a560 ffff8800d3ac7c68 ffffffff81153be7 ffff8800d3928800
[    6.411548]  ffff8800d3928970 ffff8800d3928aa0 0000000000000001 ffff8800d3ac7ca8
[    6.412780] Call Trace:
[    6.413980]  [<ffffffff81b00bbd>] dump_stack+0x4c/0x6e
[    6.415178]  [<ffffffff81153be7>] lockdep_rcu_suspicious+0xe7/0x120
[    6.416364]  [<ffffffff8179de1d>] dm_suspend+0x13d/0x140
[    6.417535]  [<ffffffff817a2d80>] ? table_load+0x340/0x340
[    6.418749]  [<ffffffff817a2f2b>] dev_suspend+0x1ab/0x260
[    6.419901]  [<ffffffff817a2d80>] ? table_load+0x340/0x340
[    6.421038]  [<ffffffff817a3781>] ctl_ioctl+0x251/0x540
[    6.422164]  [<ffffffff812b6415>] ? mntput_no_expire+0x5/0x360
[    6.423280]  [<ffffffff817a3a83>] dm_ctl_ioctl+0x13/0x20
[    6.424389]  [<ffffffff812a6f98>] do_vfs_ioctl+0x308/0x540
[    6.425515]  [<ffffffff81175d2d>] ? rcu_read_lock_held+0x6d/0x70
[    6.426601]  [<ffffffff812b324e>] ? __fget_light+0xbe/0xd0
[    6.427686]  [<ffffffff812a7251>] SyS_ioctl+0x81/0xa0
[    6.428760]  [<ffffffff81b0d6ad>] system_call_fastpath+0x16/0x1b
-- 
 Kirill A. Shutemov
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