On Wed, Aug 21, 2019 at 5:19 PM Thomas Hellström (VMware)
<thomas_os@xxxxxxxxxxxx> wrote:
> On 8/21/19 5:14 PM, Daniel Vetter wrote:
> > On Wed, Aug 21, 2019 at 5:03 PM Thomas Hellström (VMware)
> > <thomas_os@xxxxxxxxxxxx> wrote:
> >> On 8/21/19 4:47 PM, Daniel Vetter wrote:
> >>> On Wed, Aug 21, 2019 at 4:27 PM Thomas Hellström (VMware)
> >>> <thomas_os@xxxxxxxxxxxx> wrote:
> >>>> On 8/21/19 4:09 PM, Daniel Vetter wrote:
> >>>>> On Wed, Aug 21, 2019 at 2:47 PM Thomas Hellström (VMware)
> >>>>> <thomas_os@xxxxxxxxxxxx> wrote:
> >>>>>> On 8/21/19 2:40 PM, Thomas Hellström (VMware) wrote:
> >>>>>>> On 8/20/19 4:53 PM, Daniel Vetter wrote:
> >>>>>>>> With nouveau fixed all ttm-using drives have the correct nesting of
> >>>>>>>> mmap_sem vs dma_resv, and we can just lock the buffer.
> >>>>>>>>
> >>>>>>>> Assuming I didn't screw up anything with my audit of course.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> >>>>>>>> Cc: Christian Koenig <christian.koenig@xxxxxxx>
> >>>>>>>> Cc: Huang Rui <ray.huang@xxxxxxx>
> >>>>>>>> Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> >>>>>>>> Cc: "VMware Graphics" <linux-graphics-maintainer@xxxxxxxxxx>
> >>>>>>>> Cc: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> >>>>>>>> ---
> >>>>>>>> drivers/gpu/drm/ttm/ttm_bo.c | 34 ---------------------------------
> >>>>>>>> drivers/gpu/drm/ttm/ttm_bo_vm.c | 26 +------------------------
> >>>>>>>> include/drm/ttm/ttm_bo_api.h | 1 -
> >>>>>>>> 3 files changed, 1 insertion(+), 60 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> >>>>>>>> index 20ff56f27aa4..a952dd624b06 100644
> >>>>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> >>>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> >>>>>>>> @@ -1954,37 +1954,3 @@ void ttm_bo_swapout_all(struct ttm_bo_device
> >>>>>>>> *bdev)
> >>>>>>>> ;
> >>>>>>>> }
> >>>>>>>> EXPORT_SYMBOL(ttm_bo_swapout_all);
> >>>>>>>> -
> >>>>>>>> -/**
> >>>>>>>> - * ttm_bo_wait_unreserved - interruptible wait for a buffer object
> >>>>>>>> to become
> >>>>>>>> - * unreserved
> >>>>>>>> - *
> >>>>>>>> - * @bo: Pointer to buffer
> >>>>>>>> - */
> >>>>>>>> -int ttm_bo_wait_unreserved(struct ttm_buffer_object *bo)
> >>>>>>>> -{
> >>>>>>>> - int ret;
> >>>>>>>> -
> >>>>>>>> - /*
> >>>>>>>> - * In the absense of a wait_unlocked API,
> >>>>>>>> - * Use the bo::wu_mutex to avoid triggering livelocks due to
> >>>>>>>> - * concurrent use of this function. Note that this use of
> >>>>>>>> - * bo::wu_mutex can go away if we change locking order to
> >>>>>>>> - * mmap_sem -> bo::reserve.
> >>>>>>>> - */
> >>>>>>>> - ret = mutex_lock_interruptible(&bo->wu_mutex);
> >>>>>>>> - if (unlikely(ret != 0))
> >>>>>>>> - return -ERESTARTSYS;
> >>>>>>>> - if (!dma_resv_is_locked(bo->base.resv))
> >>>>>>>> - goto out_unlock;
> >>>>>>>> - ret = dma_resv_lock_interruptible(bo->base.resv, NULL);
> >>>>>>>> - if (ret == -EINTR)
> >>>>>>>> - ret = -ERESTARTSYS;
> >>>>>>>> - if (unlikely(ret != 0))
> >>>>>>>> - goto out_unlock;
> >>>>>>>> - dma_resv_unlock(bo->base.resv);
> >>>>>>>> -
> >>>>>>>> -out_unlock:
> >>>>>>>> - mutex_unlock(&bo->wu_mutex);
> >>>>>>>> - return ret;
> >>>>>>>> -}
> >>>>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>>>>>>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>>>>>>> index 76eedb963693..505e1787aeea 100644
> >>>>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>>>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>>>>>>> @@ -125,31 +125,7 @@ static vm_fault_t ttm_bo_vm_fault(struct
> >>>>>>>> vm_fault *vmf)
> >>>>>>>> &bdev->man[bo->mem.mem_type];
> >>>>>>>> struct vm_area_struct cvma;
> >>>>>>>> - /*
> >>>>>>>> - * Work around locking order reversal in fault / nopfn
> >>>>>>>> - * between mmap_sem and bo_reserve: Perform a trylock operation
> >>>>>>>> - * for reserve, and if it fails, retry the fault after waiting
> >>>>>>>> - * for the buffer to become unreserved.
> >>>>>>>> - */
> >>>>>>>> - if (unlikely(!dma_resv_trylock(bo->base.resv))) {
> >>>>>>>> - if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
> >>>>>>>> - if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
> >>>>>>>> - ttm_bo_get(bo);
> >>>>>>>> - up_read(&vmf->vma->vm_mm->mmap_sem);
> >>>>>>>> - (void) ttm_bo_wait_unreserved(bo);
> >>>>>>>> - ttm_bo_put(bo);
> >>>>>>>> - }
> >>>>>>>> -
> >>>>>>>> - return VM_FAULT_RETRY;
> >>>>>>>> - }
> >>>>>>>> -
> >>>>>>>> - /*
> >>>>>>>> - * If we'd want to change locking order to
> >>>>>>>> - * mmap_sem -> bo::reserve, we'd use a blocking reserve here
> >>>>>>>> - * instead of retrying the fault...
> >>>>>>>> - */
> >>>>>>> I think you should justify why the above code is removed, since the
> >>>>>>> comments actually outlines what to do if we change locking order.
> >>>>>>>
> >>>>>>> The code that's removed above is not for adjusting locking orders but
> >>>>>>> to decrease the mm latency by releasing the mmap_sem while waiting for
> >>>>>>> bo reserve which in turn might be waiting for GPU. At a minimum we
> >>>>>>> should have a separate patch with justification.
> >>>>>>>
> >>>>>>> Note that the caller here ensures locking progress by adjusting the
> >>>>>>> RETRY flags after a retry.
> >>>>> That would be patches 1&2 in this series.
> >>>>>
> >>>> Hmm? Those seem to touch only dma-buf and nouveau not ttm? I mean this
> >>>> patch should look along the lines of (based on an older tree) to
> >>>> implement the new locking-order mmap_sem->reservation,
> >>> Only nouveau was breaking was doing copy_*_user or get_user_pages
> >>> while holding dma_resv locks, no one else. So nothing else needed to
> >>> be changed. But patch 1 contains the full audit. I might have missed
> >>> something.
> >>>
> >>>> but to keep the mm latency optimization using the RETRY functionality:
> >>> Still no idea why this is needed? All the comments here and the code
> >>> and history seem like they've been about the mmap_sem vs dma_resv
> >>> inversion between driver ioctls and fault handling here. Once that's
> >>> officially fixed there's no reason to play games here and retry loops
> >>> - previously that was necessary because the old ttm_bo_vm_fault had a
> >>> busy spin and that's definitely not nice. If it's needed I think it
> >>> should be a second patch on top, to keep this all clear. I had to
> >>> audit an enormous amount of code, I'd like to make sure I didn't miss
> >>> anything before we start to make this super fancy again. Further
> >>> patches on top is obviously all fine with me.
> >>> -Daniel
> >> Yes, but there are two different things you remove here. One is the
> >> workaround for the locking reversal, which is obviously correct.
> >>
> >> One is TTM's implementation of the mmap_sem latency optimization, which
> >> looks like an oversight.
> >>
> >> That optimization is why the VM_FAULT_RETRY functionality was added to
> >> mm in the first place and is intended to be used when drivers expect a
> >> substantial sleep to avoid keeping the pretty globalish mmap_sem held
> >> while that sleep is taking place. We do exactly the same while waiting
> >> for move-fences (ttm_bo_vm_fault_idle) and other drivers that expect
> >> long waits in the fault handler do the same.
> > Hm, are we guaranteed that core mm will only call us once with
> > ALLOW_RETRY?
>
> Last time I looked in the implementation, yes. The ALLOW_RETRY was there
> to specifically
> allow making progress in the locking.
>
> > Just to make sure that we're not live-locking here. I'd
> > also like to get rid of the wu_mutex, that just looks really strange
> > (and I thought it was to duct-tape over the inversion, not as an
> > optimization). If the livelock due to locking inversion is gone I have
> > no idea anymore why we even needs the wu_mutex.
>
> Yes, my interpretation of this is that wu_mutex definitely can be ditched.
Ok I'll respin and just do normal interruptible locks, keeping the
mmap_sem-less path. I think perfectly ok to leave the optimization in,
as long as we can remove the wu_mutex.
btw r-b/t-b on patch 1 from vmwgfx would be very much appreciated.
That one is the real trick in this series here I think.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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