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From: "Sjur Brændeland" <sjurbren@gmail.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
	linux-kernel@vger.kernel.org, "Guzman Lugo,
	Fernadndo" <fernando.lugo@ti.com>,
	virtualization@lists.linux-foundation.org,
	Ohad Ben-Cohen <ohad@wizery.com>
Subject: Re: [PATCH] virtio: Don't access device data after unregistration.
Date: Mon, 3 Sep 2012 16:50:10 +0200	[thread overview]
Message-ID: <CAJK669aRsbOOmjmT+K8OHYvenOhkYWuSOx6nBM7hxC_C4mmw-g@mail.gmail.com> (raw)
In-Reply-To: <20120903141445.GA5054@redhat.com>

Hi Michael,

>> Fix panic in virtio.c when CONFIG_DEBUG_SLAB is set.
>
> What's the root cause of the panic?

I believe the cause of the panic is calling
ida_simple_remove(&virtio_index_ida, dev->index);
when the dev structure is "poisoned" after kfree.
It might be the "BUG_ON((int)id < 0)" that bites...

>> Use device_del() and put_device() instead of
>> device_unregister(), and access device data before
>> calling put_device().

> Why does this help? Does device_unregister free the
> device so dev->index access crashes?

Yes, if device ref-count is one when calling unregister
the device is freed.

> If yes virtio_pci_remove will crash too
> as it accesses the device after the
> call to unregister_virtio_device so the
> fix won't be effective.

I discovered this using the remoteproc framework.
It might be that device is unregistered with ref-count greater
than one normally, in that case this bug will not show up.

Regards,
Sjur

  reply	other threads:[~2012-09-03 14:50 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-09-03 13:50 sjur.brandeland
2012-09-03 14:14 ` Michael S. Tsirkin
2012-09-03 14:50   ` Sjur Brændeland [this message]
2012-09-03 20:18     ` Michael S. Tsirkin
2012-09-04 12:12       ` Sjur Brændeland
2012-09-04 14:13         ` Michael S. Tsirkin

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