Re: [RFC PATCH] drm/radeon: fixup locking inversion between mmap_semand reservations

[Thomas Hellstrom]

On 10/08/2013 04:55 PM, Jerome Glisse wrote:
> On Tue, Oct 08, 2013 at 04:45:18PM +0200, Christian König wrote:
> > Am 08.10.2013 16:33, schrieb Jerome Glisse:
> > > On Tue, Oct 08, 2013 at 04:14:40PM +0200, Maarten Lankhorst wrote:
> > > > Allocate and copy all kernel memory before doing reservations. This prevents a locking
> > > > inversion between mmap_sem and reservation_class, and allows us to drop the trylocking
> > > > in ttm_bo_vm_fault without upsetting lockdep.
> > > >
> > > > Signed-off-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
> > > I would say NAK. Current code only allocate temporary page in AGP case.
> > > So AGP case is userspace -> temp page -> cs checker -> radeon ib.
> > >
> > > Non AGP is directly memcpy to radeon IB.
> > >
> > > Your patch allocate memory memcpy userspace to it and it will then be
> > > memcpy to IB. Which means you introduce an extra memcpy in the process
> > > not something we want.
> > Totally agree. Additional to that there is no good reason to provide
> > anything else than anonymous system memory to the CS ioctl, so the
> > dependency between the mmap_sem and reservations are not really
> > clear to me.
> >
> > Christian.
> I think is that in other code path you take mmap_sem first then reserve
> bo. But here we reserve bo and then we take mmap_sem because of copy
> from user.
>
> Cheers,
> Jerome
Actually the log message is a little confusing. I think the mmap_sem 
locking inversion problem is orthogonal to what's being fixed here.

This patch fixes the possible recursive bo::reserve caused by malicious 
user-space handing a pointer to ttm memory so that the ttm fault handler 
is called when bos are already reserved. That may cause a (possibly 
interruptible) livelock.

Once that is fixed, we are free to choose the mmap_sem -> bo::reserve 
locking order. Currently it's bo::reserve->mmap_sem(), but the hack 
required in the ttm fault handler is admittedly a bit ugly.  The plan is 
to change the locking order to mmap_sem->bo::reserve

I'm not sure if it applies to this particular case, but it should be 
possible to make sure that copy_from_user_inatomic() will always 
succeed, by making sure the pages are present using get_user_pages(), 
and release the pages after copy_from_user_inatomic() is done. That way 
there's no need for a double memcpy slowpath, but if the copied data is 
very fragmented I guess the resulting code may look ugly. The 
get_user_pages() function will return an error if it hits TTM pages.

/Thomas
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