linux-kernel - Re: [V9fs-developer] Hang triggered by udev coldplug, looks like a race

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Date:	Thu, 24 Dec 2015 11:51:49 +0100
From:	Dominique Martinet <dominique.martinet@....fr>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [V9fs-developer] Hang triggered by udev coldplug, looks like a
 race

Andy Lutomirski wrote on Thu, Dec 17, 2015:
> This could be QEMU's analysis script screwing up.  Is there a good way
> for me to gather more info?

I finally took some time to reproduce it (sorry for the delay)

Using your config, virtme commit (17363c2) and kernel tag v4.4-rc3 I was
able to reproduce it just fine with my qemu (2.4.90)

Now for the fun bit... I ran it with a gdb server, attaching gdb and
running cont always 'unblocks' it
Using the kernel gdb scripts (lx-ps) I see about 250 kworker threads
running, the backtraces all look the same:

[   20.273945]  [<ffffffff818c3910>] schedule+0x30/0x80
[   20.274644]  [<ffffffff818c3c39>] schedule_preempt_disabled+0x9/0x10
[   20.275539]  [<ffffffff818c6147>] __mutex_lock_slowpath+0x107/0x2f0
[   20.276421]  [<ffffffff811cf02e>] ? lookup_fast+0xbe/0x320
[   20.277195]  [<ffffffff818c6345>] mutex_lock+0x15/0x30
[   20.277916]  [<ffffffff811d0df7>] walk_component+0x1a7/0x270

so given it unblocks after hooking gdb + cont I'm actually thinking this
might be a pure scheduling issue? (e.g. thread is never re-scheduled or
something like that?)
I can't see any task not in schedule() in your sysrq dump task
transcript either.

Not sure how to go around debugging that, to be honest.
I've tried both default one virtual cpu and -smp 3 or 4 and both can
reproduce it; cpu usage on the host is always low so it doesn't look
like there's any busy-polling involved.. This is a pretty subtle bug we
have there..

-- 
Dominique Martinet
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