[Xen-devel] Re: hang in pvfb resulting from save/restore?

[Markus Armbruster <armbru@xxxxxxxxxx>]

Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:

> I suspect there's a bug in xen-pvfb, possibly triggered by save/restore.
>
> If I run X on pvfb, running a couple of instances of something busy
> like glxgears, and then do a few rounds of save/restore, one of my
> "events" kernel threads goes into 100% CPU spin and X stops
> responding.  I'm not sure what it's doing, but after a while the
> softlockup detector triggers:
>
> INFO: task X:3408 blocked for more than 240 seconds.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> X             D cf5f7da0     0  3408   3403
>       cf5f7de4 00000202 00000000 cf5f7da0 c05c3400 cf5f7da8 c01022b3
> ca838000     ca838268 c184fa80 00000000 00000040 cf5f6000 ca838000
> 0003909e c0149d2c     c025a8c3 00000000 00000000 00000000 ffffffff
> c05d8288 c05d8284 00000200 Call Trace:
> [<c01022b3>] ? xen_restore_fl+0x2e/0x52
> [<c0149d2c>] ? lock_contended+0x15a/0x16f
> [<c025a8c3>] ? fb_deferred_io_mkwrite+0x23/0x56
> [<c045e9de>] mutex_lock_nested+0x17d/0x296
> [<c025a8c3>] ? fb_deferred_io_mkwrite+0x23/0x56
> [<c025a8c3>] fb_deferred_io_mkwrite+0x23/0x56
> [<c0164bb9>] do_wp_page+0xdc/0x6bc
> [<c01022b3>] ? xen_restore_fl+0x2e/0x52
> [<c0149ebc>] ? lock_acquired+0x17b/0x194
> [<c016826b>] handle_mm_fault+0xa2b/0xb36
> [<c010c9a4>] ? restore_i387+0xeb/0x138
> [<c01022b3>] ? xen_restore_fl+0x2e/0x52
> [<c014bb2b>] ? lock_acquire+0x99/0xa6
> [<c011d1eb>] ? do_page_fault+0x433/0x934
> [<c0142089>] ? down_read_trylock+0x37/0x41
> [<c011d29f>] do_page_fault+0x4e7/0x934
> [<c0105ac1>] ? restore_sigcontext+0x14d/0x1cb
> [<c0148ca7>] ? trace_hardirqs_off+0xb/0xd
> [<c01022b3>] ? xen_restore_fl+0x2e/0x52
> [<c0148ca7>] ? trace_hardirqs_off+0xb/0xd
> [<c011cdb8>] ? do_page_fault+0x0/0x934
> [<c04607c2>] error_code+0x72/0x78

Implicates drivers/video/fb_defio.c (author cc'ed).  Jaya, any ideas?

> =======================
> INFO: lockdep is turned off.
>
> The rest of the system is working OK (though I expect things are
> getting queued up on events/1).
>
> I haven't dug in to really see what the problem is, but given that I
> just implemented save/restore, it seems likely that pvfb's
> save/restore handling will be a bit untested ;)

Ha, so it's your bug then!  ;->

> Other info:
>
> X's wchan is fb_deferred_io_mkwrite
>
> I can't work out where events/1 is spinning.  xenctx shows the eip is
> xen_irq_disable, apparently unchanging; xenctx doesn't seem to be able
> to read the stack, so I don't have any context.
>
> ...and now the whole vm has locked up, so I can't investigate more.
>
>    J

Okay, thanks!

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