Messages in this thread | | | Date | Tue, 08 Oct 2013 18:29:35 +0200 | From | Thomas Hellstrom <> | Subject | Re: [RFC PATCH] drm/radeon: fixup locking inversion between mmap_sem and reservations |
| |
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@canonical.com> >>> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |