[RFC PATCH] drm/nouveau: fix nested locking in mmap handler

[Maarten Lankhorst]

Op 24-09-13 11:03, Thomas Hellstrom schreef:
> On 09/24/2013 09:34 AM, Maarten Lankhorst wrote:
>> Op 24-09-13 09:22, Thomas Hellstrom schreef:
>>> On 09/23/2013 05:33 PM, Maarten Lankhorst wrote:
>>>> Hey,
>>>>
>>>> Op 13-09-13 11:00, Peter Zijlstra schreef:
>>>>> On Fri, Sep 13, 2013 at 10:41:54AM +0200, Daniel Vetter wrote:
>>>>>> On Fri, Sep 13, 2013 at 10:29 AM, Peter Zijlstra <peterz at infradead.org> wrote:
>>>>>>> On Fri, Sep 13, 2013 at 09:46:03AM +0200, Thomas Hellstrom wrote:
>>>>>>>>>> if (!bo_tryreserve()) {
>>>>>>>>>>       up_read mmap_sem(); // Release the mmap_sem to avoid deadlocks.
>>>>>>>>>>       bo_reserve();               // Wait for the BO to become available (interruptible)
>>>>>>>>>>       bo_unreserve();           // Where is bo_wait_unreserved() when we need it, Maarten :P
>>>>>>>>>>       return VM_FAULT_RETRY; // Go ahead and retry the VMA walk, after regrabbing
>>>>>>>>>> }
>>>>>>>> Anyway, could you describe what is wrong, with the above solution, because
>>>>>>>> it seems perfectly legal to me.
>>>>>>> Luckily the rule of law doesn't have anything to do with this stuff --
>>>>>>> at least I sincerely hope so.
>>>>>>>
>>>>>>> The thing that's wrong with that pattern is that its still not
>>>>>>> deterministic - although its a lot better than the pure trylock. Because
>>>>>>> you have to release and re-acquire with the trylock another user might
>>>>>>> have gotten in again. Its utterly prone to starvation.
>>>>>>>
>>>>>>> The acquire+release does remove the dead/life-lock scenario from the
>>>>>>> FIFO case, since blocking on the acquire will allow the other task to
>>>>>>> run (or even get boosted on -rt).
>>>>>>>
>>>>>>> Aside from that there's nothing particularly wrong with it and lockdep
>>>>>>> should be happy afaict (but I haven't had my morning juice yet).
>>>>>> bo_reserve internally maps to a ww-mutex and task can already hold
>>>>>> ww-mutex (potentially even the same for especially nasty userspace).
>>>>> OK, yes I wasn't aware of that. Yes in that case you're quite right.
>>>>>
>>>> I added a RFC patch below.  I only tested with PROVE_LOCKING, and always forced the slowpath for debugging.
>>>>
>>>> This fixes nouveau and core ttm to always use blocking acquisition in fastpath.
>>>> Nouveau was a bit of a headache, but afaict it should work.
>>>>
>>>> In almost all cases relocs are not updated, so I kept intact the fastpath
>>>> of not copying relocs from userspace. The slowpath tries to copy it atomically,
>>>> and if that fails it will unreserve all bo's and copy everything.
>>>>
>>>> One thing to note is that the command submission ioctl may fail now with -EFAULT
>>>> if presumed cannot be updated, while the commands are submitted succesfully.
>>> I think the Nouveau guys need to comment further on this, but returning -EFAULT might break existing user-space, and that's not allowed, but IIRC the return value of "presumed" is only a hint, and if it's incorrect will only trigger future command stream patching.
>>>
>>> Otherwise reviewing mostly the TTM stuff. FWIW, from wat I can tell the vmwgfx driver doesn't need any fixups.
>> Well because we read the list of buffer objects the presumed offsets are at least read-mapped. Although I guess in the worst case the mapping might disappear before the syscall copies back the data.
>> So if -EFAULT happens here then userspace messed up in some way, either by forgetting to map the offsets read-write, which cannot happen with libdrm or free'ing the bo list before the syscall returns,
>> which would probably result in libdrm crashing shortly afterwards anyway.
>
> Hmm, is the list of buffer objects (and the "presumed" members) really in DRM memory? Because if it resides or may reside in anonymous system memory, it may well be paged out between reading and writing, in which case the -EFAULT return is incorrect.
It may be swapped out, but I don't use read.*atomic copy calls, faults are allowed to happen at that point and no -EFAULT will be returned from
that. The only -EFAULT that can happen is if the memory ends up being read-only, or ceased to exist.

> In fact failures of pushbuf / execbuf *after* commands are successfully submitted are generally very hard to recover from. That's why the kernel should do whatever it takes to recover from such failures, and user-space should do whatever it takes to recover from copy-to-user failures of needed info from the kernel, and it really depends on the user-space usage pattern of "presumed". IIRC the original reason for copying it back to user-space was, that if a relocation offsets were patched up by the kernel, and then the process was sent a signal causing it to retry execbuf, then "presumed" had to be updated, otherwise it would be inconsistent with what's currently in the command stream, which is very bad. If "presumed" is, however, only used by user-space to guess an offset, the correct action would be to swallow the -EFAULT.
Yeah it's a guess from userspace. But it's probably still a programming error by userspace if it uses a read-only
mapping for presumed, or a mapping that's invalidated. :P But there's nothing I can do at that point,
I can't undo something I just committed. This is why I asked whether I should swallow it or not, because it's
unclear to me. :) There's not much of an impact to userspace if I swallow it, but userspace did deliberately
mess up the call.

>>
>> So I don't know whether to swallow that -EFAULT or not, which is what I asked.
>>
>> And for vmwgfx just run it through PROVE_LOCKING and make sure all the copy_to/from_user call sites are called at least once, and then you can be certain it doesn't need fixups. ;)
>> Lockdep ftw..
> Will do that.
>
>>
>>> Actually, it's not the locking order bo:reserve -> mmap_sem that triggers the lockdep warning, right? but the fact that copy_from_user could recurse into the fault handler? Grabbing bo:reseves recursively? which should leave us free to choose locking order at this point.
>> Same thing.
>>
>> When PROVE_LOCKING is enabled the might_fault calls in copy_to/from_user do a fake locking of mmap_sem, which means all locking errors, provided that the reservation lock is taken at least once with mmap_sem held (eg the ttm fault handler is called at least once, it can be done separately, it doesn't need to be done in the same syscall). So any bugs will be found. The only thing to worry about is that lockdep turns off after finding 1 error, so you have to make sure it doesn't bomb out before completing tests, which is sometimes a problem on early rc kernels. ;)
>
> My point was that when we only have copy_[to|from]_user_inatomic inside any bo:reservations, the might_fault would never be called inside any reservations and we should, in principle, be free to choose locking order, provided of course it could be done cleanly in fault()?
I think it makes sense to keep mmap_sem the outer lock here, and not do scary things in fault. Especially if the mm locking is going to be changed in the future. But you're correct, if holding reservations only inatomic should be used.

~Maarten