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Re: [Xen-devel] [PATCH] [PVOPS] fix gntdev on PAE

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] [PVOPS] fix gntdev on PAE
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 28 May 2010 10:29:32 -0700
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Gerd Hoffmann <kraxel@xxxxxxxxxx>
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On 02/10/2010 04:19 AM, Stefano Stabellini wrote:
> On Tue, 9 Feb 2010, Jeremy Fitzhardinge wrote:
>> On 02/01/2010 07:46 AM, Stefano Stabellini wrote:
>>> On Mon, 1 Feb 2010, Stefano Stabellini wrote:
>>>> Hi all,
>>>> this small patch fixes gntdev on Linux pvops kernels:
>>>> gnttab_set_map_op and gnttab_set_unmap_op shouldn't take unsigned long
>>>> as parameters for machine addresses because they are not big enough on
>>>> PAE systems.
>>>> This patch fixes the issue using phys_addr_t instead and enables
>>>> XEN_GNTDEV compilation again.
>>>> Signed-off-by: Stefano Stabellini<stefano.stabellini@xxxxxxxxxxxxx>
>>> BTW gntdev is used by qemu to provide the console backend to pv guests.
>> Is that recent?  Console had been working before hadn't it?
>> The gntdev problems I saw were more locking related than anything to do 
>> with PAE.  Did you try testing with lock debugging enabled?
> Yes, I don't have any problem with locking in gntdev on my testbox.

I managed to catch a lockdep problem in gntdev, which may be the same as

BUG: sleeping function called from invalid context at kernel/rwsem.c:21
in_atomic(): 1, irqs_disabled(): 0, pid: 4091, name: qemu-dm
2 locks held by qemu-dm/4091:
 #0:  (&mm->mmap_sem){++++++}, at: [<ffffffff810bb50f>] sys_munmap+0x33/0x58
 #1:  (rcu_read_lock){.+.+..}, at: [<ffffffff810cd63a>] 
Pid: 4091, comm: qemu-dm Not tainted #23
Call Trace:
 [<ffffffff8106705b>] ? __debug_show_held_locks+0x22/0x24
 [<ffffffff81039522>] __might_sleep+0x123/0x127
 [<ffffffff810a8536>] ? release_pages+0xd2/0x1e7
 [<ffffffff81498849>] down_read+0x1f/0x57
 [<ffffffff81010142>] ? check_events+0x12/0x20
 [<ffffffff810a8536>] ? release_pages+0xd2/0x1e7
 [<ffffffff810cd63a>] ? __mmu_notifier_invalidate_range_start+0x0/0xc7
 [<ffffffff8123e069>] mn_invl_range_start+0x32/0x118
 [<ffffffff810cd69c>] __mmu_notifier_invalidate_range_start+0x62/0xc7
 [<ffffffff810cd63a>] ? __mmu_notifier_invalidate_range_start+0x0/0xc7
 [<ffffffff810b54bc>] unmap_vmas+0x8c/0x91a
 [<ffffffff810ba363>] unmap_region+0xda/0x178
 [<ffffffff810bb472>] do_munmap+0x2ae/0x318
 [<ffffffff810bb51d>] sys_munmap+0x41/0x58
 [<ffffffff81013b82>] system_call_fastpath+0x16/0x1b

The problem is that mn_invl_range_start does a down_read(), but it is
called from __mmu_notifier_invalidate_range_start(), which does an
rcu_read_lock, which has the side-effect of disabling preemption.

The mmu notifier code seems to have always used rcu_read_lock this way,
so I guess this bug has always been there.  It's not immediately obvious
how to fix it.



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