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SubjectRe: [PATCH 4/6] nouveau: unlock mmap_sem on all errors from nouveau_range_fault
On Tue, Jul 23, 2019 at 06:30:48PM +0200, Christoph Hellwig wrote:
> On Tue, Jul 23, 2019 at 03:18:28PM +0000, Jason Gunthorpe wrote:
> > Hum..
> >
> > The caller does this:
> >
> > again:
> > ret = nouveau_range_fault(&svmm->mirror, &range);
> > if (ret == 0) {
> > mutex_lock(&svmm->mutex);
> > if (!nouveau_range_done(&range)) {
> > mutex_unlock(&svmm->mutex);
> > goto again;
> >
> > And we can't call nouveau_range_fault() -> hmm_range_fault() without
> > holding the mmap_sem, so we can't allow nouveau_range_fault to unlock
> > it.
>
> Goto again can only happen if nouveau_range_fault was successful,
> in which case we did not drop mmap_sem.

Oh, right we switch from success = number of pages to success =0..

However the reason this looks so weird to me is that the locking
pattern isn't being followed, any result of hmm_range_fault should be
ignored until we enter the svmm->mutex and check if there was a
colliding invalidation.

So the 'goto again' *should* be possible even if range_fault failed.

But that is not for this patch..

> > ret = hmm_range_fault(range, true);
> > if (ret <= 0) {
> > if (ret == 0)
> > ret = -EBUSY;
> > - up_read(&range->vma->vm_mm->mmap_sem);
> > hmm_range_unregister(range);
>
> This would hold mmap_sem over hmm_range_unregister, which can lead
> to deadlock if we call exit_mmap and then acquire mmap_sem again.

That reminds me, this code is also leaking hmm_range_unregister() in
the success path, right?

I think the right way to structure this is to move the goto again and
related into the nouveau_range_fault() so the whole retry algorithm is
sensibly self contained.

Jason

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