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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 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.

Also:

> 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.

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