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

[Daniel Vetter]

On Tue, Sep 24, 2013 at 11:03:37AM +0200, Thomas Hellstrom wrote:
> 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@xxxxxxxxxxxxx> 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.
> 
> 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.

In i915 we've had tons of fun with a regression in 3.7 where exactly this
blows up: Some of our userspace (UXA ddx specifically) retains
relocations-trees partially accross an execbuf. Which means if the kernel
updates the relocations it _must_ update the presumed offset for
otherwise things will go haywire on the next execbuf. So we can't return
-EFAULT if the userspace memory needs to be just refaulted but still need
to guarante a "correct" presumed offset.

Since we didn't come up with a way to make sure this will work in all
cases when we get an -EFAULT when writing back presumed offsets we have a
rather tricky piece of fallback logic.
- Any -EFAULT error in the fastpath will drop us into the relocation
  slowpath. The slowpath completly processes relocs anew, so any updates
  done by the fastpath are irrelevant.

- The first thing the slowpath does is set the presumed offset in the
  userspace reloc lists to an invalid address (we pick -1) to make sure
  that any subsequent execbuf with the same partial reloc tree will again
  go through the relocation update code.

- Then we do the usual slowpath, i.e. copy relocs from userspace, re-grab
  locks and then process them. The copy-back of the presumed offset
  happens with an copy_to_user_inatomic, and we ignore any errors.

Of course we try really hard to make sure that we never hit the reloc
slowpath ;-) But nowadays this is all fully tested with some nasty
testcases (and a small kernel option to disable prefaulting).

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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