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SubjectRe: [PATCH 4/6] nouveau: unlock mmap_sem on all errors from nouveau_range_fault
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On Mon, Jul 22, 2019 at 11:44:24AM +0200, Christoph Hellwig wrote:
> Currently nouveau_svm_fault expects nouveau_range_fault to never unlock
> mmap_sem, but the latter unlocks it for a random selection of error
> codes. Fix this up by always unlocking mmap_sem for non-zero return
> values in nouveau_range_fault, and only unlocking it in the caller
> for successful returns.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> drivers/gpu/drm/nouveau/nouveau_svm.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
> index 5dd83a46578f..5de2d54b9782 100644
> +++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
> @@ -494,8 +494,10 @@ nouveau_range_fault(struct hmm_mirror *mirror, struct hmm_range *range)
> ret = hmm_range_register(range, mirror,
> range->start, range->end,
> PAGE_SHIFT);
> - if (ret)
> + if (ret) {
> + up_read(&range->vma->vm_mm->mmap_sem);
> return (int)ret;
> + }
>
> if (!hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT)) {
> up_read(&range->vma->vm_mm->mmap_sem);
> @@ -504,11 +506,9 @@ nouveau_range_fault(struct hmm_mirror *mirror, struct hmm_range *range)
>
> ret = hmm_range_fault(range, true);
> if (ret <= 0) {
> - if (ret == -EBUSY || !ret) {
> - up_read(&range->vma->vm_mm->mmap_sem);
> - ret = -EBUSY;
> - } else if (ret == -EAGAIN)
> + if (ret == 0)
> ret = -EBUSY;
> + up_read(&range->vma->vm_mm->mmap_sem);
> hmm_range_unregister(range);
> return ret;

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.

Maybe this instead?

diff --git a/drivers/gpu/drm/nouveau/nouveau_svm.c b/drivers/gpu/drm/nouveau/nouveau_svm.c
index a9c5c58d425b3d..92cf760a9bcc5d 100644
--- a/drivers/gpu/drm/nouveau/nouveau_svm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_svm.c
@@ -494,21 +494,16 @@ nouveau_range_fault(struct hmm_mirror *mirror, struct hmm_range *range)
ret = hmm_range_register(range, mirror,
range->start, range->end,
PAGE_SHIFT);
- if (ret) {
- up_read(&range->vma->vm_mm->mmap_sem);
+ if (ret)
return (int)ret;
- }

- if (!hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT)) {
- up_read(&range->vma->vm_mm->mmap_sem);
+ if (!hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT))
return -EBUSY;
- }

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);
return ret;
}
@@ -706,8 +701,8 @@ nouveau_svm_fault(struct nvif_notify *notify)
NULL);
svmm->vmm->vmm.object.client->super = false;
mutex_unlock(&svmm->mutex);
- up_read(&svmm->mm->mmap_sem);
}
+ up_read(&svmm->mm->mmap_sem);

/* Cancel any faults in the window whose pages didn't manage
* to keep their valid bit, or stay writeable when required.
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