Re: [PATCH 4.21 V3] blk-mq: not embed .mq_kobj and ctx->kobj into queue instance

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Nov 20, 2018 at 08:51:50AM +0100, Greg Kroah-Hartman wrote:
> On Tue, Nov 20, 2018 at 09:44:35AM +0800, Ming Lei wrote:
> > Even though .mq_kobj, ctx->kobj and q->kobj share same lifetime
> > from block layer's view, actually they don't because userspace may
> > grab one kobject anytime via sysfs.
> > 
> > This patch fixes the issue by the following approach:
> > 
> > 1) introduce 'struct blk_mq_ctxs' for holding .mq_kobj and managing
> > all ctxs
> > 
> > 2) free all allocated ctxs and the 'blk_mq_ctxs' instance in release
> > handler of .mq_kobj
> > 
> > 3) grab one ref of .mq_kobj before initializing each ctx->kobj, so that
> > .mq_kobj is always released after all ctxs are freed.
> > 
> > This patch fixes kernel panic issue during booting when DEBUG_KOBJECT_RELEASE
> > is enabled.
> > 
> > Reported-by: Guenter Roeck <linux@roeck-us.net>
> > Cc: "jianchao.wang" <jianchao.w.wang@oracle.com>
> > Cc: Guenter Roeck <linux@roeck-us.net>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> > V3:
> > 	- keep to allocate q->queue_ctx via percpu allocator, so one extra
> > 	  pointer reference can be saved for getting ctx
> > V2:
> > 	- allocate 'blk_mq_ctx' inside blk_mq_init_allocated_queue()
> > 	- allocate q->mq_kobj directly 
> 
> Not tested, but seems sane from a kobject point-of-view:
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Tested-by: Guenter Roeck <linux@roeck-us.net>

with v4.14.y and v4.19.y.

The patch is marked for v4.21. I would kindly suggest to not wait for v4.21
but apply it to v4.20. This would let us enable DEBUG_KOBJECT_RELEASE
with syzbot on upstream and stable kernels.

Greg, applying the patch to v4.14.y will require a backport due to a minor
context conflict. I'll send that to you after the patch is available in
mainline.

Thanks,
Guenter