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Date: Wed, 10 Dec 2014 17:03:25 +0100
Message-ID: <s5h1to7o5ki.wl-tiwai@suse.de>
From: Takashi Iwai <tiwai@suse.de>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] blk-mq: Fix uninitialized kobject at CPU hotplugging
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References: <1418225910-22108-1-git-send-email-tiwai@suse.de>
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At Wed, 10 Dec 2014 09:01:05 -0700,
Jens Axboe wrote:
> 
> 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.

Maybe you did test CPU hot removal before adding?


Takashi