Re: Double gnttab_end_access in mini-os netfront

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, Dec 08, 2015 at 11:33:49AM +0000, Ian Campbell wrote:
> On Sun, 2015-12-06 at 03:19 +0100, Marek Marczykowski-Górecki wrote:
> > Hi,
> > 
> > When running HVM on Xen 4.6 with qemu in stubdom, I've found that
> > something goes wrong with disk frontend (at least that was visible
> > problem - a lot of read and write errors in stubdom log). But further
> > debugging (including -DGNT_DEBUG) leads to double gnttab_end_access in
> > netfront. 
> > 
> > The stack trace is:
> > ASSERTION FAILED: !(!inuse[ref]) at gnttab.c:42.
> > Do_exit called!
> > 0x00000000000f3ffb: get_time_values_from_xen at xen-4.6.0/extras/mini-
> > os/arch/x86/time.c:134 (discriminator 1)
> > 0x00000000000d74a8: HYPERVISOR_sched_op at xen-4.6.0/extras/mini-
> > os/include/x86/x86_64/hypercall-x86_64.h:180
> > 0x00000000000d6a2e: put_free_entry at xen-4.6.0/extras/mini-
> > os/gnttab.c:43
> > 0x00000000000d6bff: gnttab_end_access at xen-4.6.0/extras/mini-
> > os/gnttab.c:115
> > 0x00000000000d8a50: network_rx at xen-4.6.0/extras/mini-os/netfront.c:134
> > 0x00000000000d9ec4: netfront_receive at xen-4.6.0/extras/mini-
> > os/netfront.c:671
> > 0x00000000000dd302: get_current at xen-4.6.0/extras/mini-
> > os/include/x86/arch_sched.h:16
> > 0x0000000000079f72: tap_send at xen-4.6.0/stubdom/../tools/qemu-xen-
> > traditional/net.c:756
> > 0x00000000000069f9: main_loop_wait at xen-4.6.0/stubdom/../tools/qemu-
> > xen-traditional/vl.c:3826
> > 0x0000000000021f27: main_loop at xen-4.6.0/stubdom/../tools/qemu-xen-
> > traditional/i386-dm/helper2.c:612 (discriminator 1)
> > 0x000000000000950d: quit_timers at xen-4.6.0/stubdom/../tools/qemu-xen-
> > traditional/vl.c:1866
> > 0x00000000000d7f57: call_main at xen-4.6.0/extras/mini-os/main.c:163
> > 0x0000000000003423: thread_starter at :?
> > 0x0000000000000000: _start at ??:?
> > 
> > It was working in Xen 4.4. The only commit touching xenfront.c (in any
> > meaningful way) from that time is this one:
> > http://xenbits.xen.org/gitweb/?p=mini-os.git;a=commit;h=7c8f348390652a67e
> > 9356eec9cd2b0f76a9c7c72
> > 
> > With that commit reverted, issue vanishes.
> > 
> > I guess it's because before this commit, there was "if (rx->status ==
> > NETIF_RSP_NULL) continue" before "gnttab_end_access(buf->gref)", but now
> > that case is handled after gnttab_end_access (using "if (rx->status >
> > NETIF_RSP_NULL)"). I think the fix would be to restore that "continue"
> > line.
> 
> That sounds pretty plausible to me (FWIW). Have you tried it?

I've tried moving gnttab_end_access into that if branch. And it didn't
worked.

> > PS What is the correct place for such reports? minios-devel? xen-devel?
> > both?
> 
> Formally I suppose the former, but realistically including xen-devel as
> well is likely to attract more eyes.
> 
> Ian.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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