Re: [PATCH] blk-mq: Fix uninitialized kobject at CPU hotplugging

[Jens Axboe]

On 12/10/2014 08:38 AM, Takashi Iwai wrote:
> When a CPU is hotplugged, the current blk-mq spews a warning like:
>
>    kobject '(null)' (ffffe8ffffc8b5d8): tried to add an uninitialized object, something is seriously wrong.
>    CPU: 1 PID: 1386 Comm: systemd-udevd Not tainted 3.18.0-rc7-2.g088d59b-default #1
>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_171129-lamiak 04/01/2014
>     0000000000000000 0000000000000002 ffffffff81605f07 ffffe8ffffc8b5d8
>     ffffffff8132c7a0 ffff88023341d370 0000000000000020 ffff8800bb05bd58
>     ffff8800bb05bd08 000000000000a0a0 000000003f441940 0000000000000007
>    Call Trace:
>     [<ffffffff81005306>] dump_trace+0x86/0x330
>     [<ffffffff81005644>] show_stack_log_lvl+0x94/0x170
>     [<ffffffff81006d21>] show_stack+0x21/0x50
>     [<ffffffff81605f07>] dump_stack+0x41/0x51
>     [<ffffffff8132c7a0>] kobject_add+0xa0/0xb0
>     [<ffffffff8130aee1>] blk_mq_register_hctx+0x91/0xb0
>     [<ffffffff8130b82e>] blk_mq_sysfs_register+0x3e/0x60
>     [<ffffffff81309298>] blk_mq_queue_reinit_notify+0xf8/0x190
>     [<ffffffff8107cfdc>] notifier_call_chain+0x4c/0x70
>     [<ffffffff8105fd23>] cpu_notify+0x23/0x50
>     [<ffffffff81060037>] _cpu_up+0x157/0x170
>     [<ffffffff810600d9>] cpu_up+0x89/0xb0
>     [<ffffffff815fa5b5>] cpu_subsys_online+0x35/0x80
>     [<ffffffff814323cd>] device_online+0x5d/0xa0
>     [<ffffffff81432485>] online_store+0x75/0x80
>     [<ffffffff81236a5a>] kernfs_fop_write+0xda/0x150
>     [<ffffffff811c5532>] vfs_write+0xb2/0x1f0
>     [<ffffffff811c5f42>] SyS_write+0x42/0xb0
>     [<ffffffff8160c4ed>] system_call_fastpath+0x16/0x1b
>     [<00007f0132fb24e0>] 0x7f0132fb24e0
>
> This is indeed because of an uninitialized kobject for blk_mq_ctx.
> The blk_mq_ctx kobjects are initialized in blk_mq_sysfs_init(), but it
> goes loop over hctx_for_each_ctx(), i.e. it initializes only for
> online CPUs.  Thus, when a CPU is hotplugged, the ctx for the newly
> onlined CPU is registered without initialization.
>
> This patch fixes the issue by initializing the all ctx kobjects
> belonging to each queue.

Thanks, this looks good. I wonder why I haven't hit this yet.


--
Jens Axboe

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