linux-kernel - Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock

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Date:	Tue, 13 Jul 2010 16:23:20 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Palfrader <peter@...frader.org>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Glauber Costa <glommer@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization	point
 for pvclock

On 07/13/2010 01:23 PM, Peter Palfrader wrote:
> Sorry for the delay.
>    

A good trace is worth it.

>   [<ffffffff810373c0>] ? child_rip+0x0/0x20
> Code: 78 4f 44 89 e1 48 d3 e0 44 89 f2 48 f7 e2 48 0f ac d0 20 4a 8d 0c 38 44 39 03 75 a4 48 8b 15 15 df 6b 00 48 39 ca 77 17 48 89 d0<f0>  48 0f b1 0d 04 df 6b 00 48 39 c1 48 89 c2 75 e7 48 89 ca 48
> RIP  [<ffffffff81054f73>] pvclock_clocksource_read+0x83/0xc0
>   RSP<ffff88007fb47880>
> CR2: ffffffff81712e80
> ---[ end trace 38f25e02ae34d4c1 ]---
> BUG: unable to handle kernel paging request at ffffffff81712e80
> IP: [<ffffffff81054f73>] pvclock_clocksource_read+0x83/0xc0
> PGD 1003067 PUD 1007063 PMD 7e8b1063 PTE 8000000001712161
>    

Read-only pte

> Oops: 0003 [#2] SMP
>    

Write fault.

> Code: 78 4f 44 89 e1 48 d3 e0 44 89 f2 48 f7 e2 48 0f ac d0 20 4a 8d 0c 38 44 39 03 75 a4 48 8b 15 15 df 6b 00 48 39 ca 77 17 48 89 d0<f0>  48 0f b1 0d 04 df 6b 00 48 39 c1 48 89 c2 75 e7 48 89 ca 48
> RIP  [<ffffffff81054f73>] pvclock_clocksource_read+0x83/0xc0
>    

--> lock cmpxchg %rcx,0x6bdf04(%rip)

> static atomic64_t last_value = ATOMIC64_INIT(0);
>
> cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
> {
>

[...]

>     last = atomic64_read(&last_value);
>     do {
>         if (ret < last)
>             return last;
>         last = atomic64_cmpxchg(&last_value, last, ret);
>     } while (unlikely(last != ret));
>
>     return ret;
> }

So it looks like last_value was placed in a read only section.  Please 
post your System.map somewhere.

-- 
error compiling committee.c: too many arguments to function

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