Re: [PATCH block/for-3.4/core] cfq: fix cfqg ref handling when BLK_CGROUP && !CFQ_GROUP_IOSCHED

Jens Axboe
2012-03-23T13:05:13Z

On 03/15/2012 05:35 PM, Tejun Heo wrote:
> When BLK_CGROUP is enabled but CFQ_GROUP_IOSCHED is, cfq ends up
> calling blkg_get/put() on dummy cfqg leading to the following crash.
>
> BUG: unable to handle kernel NULL pointer dereference at 00000000000000b0
> IP: [<ffffffff813d44d8>] cfq_init_queue+0x258/0x430
> PGD 0
> Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> CPU 0
> Modules linked in:
>
> Pid: 1, comm: swapper/0 Not tainted 3.3.0-rc6-work+ #125 Bochs Bochs
> RIP: 0010:[<ffffffff813d44d8>] [<ffffffff813d44d8>] cfq_init_queue+0x258/0x430
> RSP: 0018:ffff88001f9dfd80 EFLAGS: 00010046
> RAX: ffff88001aefbbf0 RBX: ffff88001aeedbf0 RCX: 0000000000000100
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff820ffd40
> RBP: ffff88001f9dfdd0 R08: 0000000000000000 R09: 0000000000000001
> R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000009 R14: ffff88001aefbc30 R15: 0000000000000003
> FS: 0000000000000000(0000) GS:ffff88001fc00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> CR2: 00000000000000b0 CR3: 000000000206f000 CR4: 00000000000006f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Process swapper/0 (pid: 1, threadinfo ffff88001f9de000, task ffff88001f9dc040)
> Stack:
> ffff88001aeedbf0 ffff88001aefbdb0 ffff88001aef1548 ffff88001aefbbf0
> ffff88001f9dfdd0 ffff88001aef1548 ffffffff820d6320 ffffffff8165ce30
> ffffffff82c555e0 ffff88001aeebbf0 ffff88001f9dfe00 ffffffff813b0507
> Call Trace:
> [<ffffffff813b0507>] elevator_init+0xd7/0x140
> [<ffffffff813b83d5>] blk_init_allocated_queue+0x125/0x150
> [<ffffffff813b94d3>] blk_init_queue_node+0x43/0x80
> [<ffffffff813b9523>] blk_init_queue+0x13/0x20
> [<ffffffff821aec00>] floppy_init+0x82/0xec7
> [<ffffffff810001d2>] do_one_initcall+0x42/0x170
> [<ffffffff821835fc>] kernel_init+0xcb/0x14f
> [<ffffffff81b40b24>] kernel_thread_helper+0x4/0x10
> Code: 00 e8 1d 9e 76 00 48 8b 43 48 48 85 c0 48 89 83 28 03 00 00 74 07 4c 8b a0 10 ff ff ff 8b 15 b0 2e d0 00 85 d2 0f 85 49 01 00 00 <41> 8b 84 24 b0 00 00 00 85 c0 0f 8e 8c 01 00 00 83 e8 01 85 c0
> RIP [<ffffffff813d44d8>] cfq_init_queue+0x258/0x430
>
> Because cfq's blkcg support has a on/off switch, CFQ_GROUP_IOSCHED,
> separate from BLK_CGROUP, blkg access through cfqg needs to be
> conditioned on it.
>
> * Make blkg_to_cfqg() and cfqg_to_blkg() conditioned on
> CFQ_GROUP_IOSCHED. If disabled, they always return %NULL.
>
> * Introduce cfqg_get() and cfqg_put() conditioned on
> CFQ_GROUP_IOSCHED. If disabled, they are noops.
>
> Reported-by: Fengguang Wu <fengguang.wu@intel.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> Yeap, forgot to test that config combination. Fengguang, can you
> please test this patch? Jens, once Fengguang confirms the fix, can
> you please apply this on top of for-3.4/core along with the pending
> stats updates?

Added to for-3.5/core, thanks.

--
Jens Axboe