On 8/21/19 5:14 PM, Daniel Vetter wrote:
On Wed, Aug 21, 2019 at 5:03 PM Thomas Hellström (VMware) thomas_os@shipmail.org 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@shipmail.org 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@shipmail.org 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@intel.com >> Cc: Christian Koenig christian.koenig@amd.com >> Cc: Huang Rui ray.huang@amd.com >> Cc: Gerd Hoffmann kraxel@redhat.com >> Cc: "VMware Graphics" linux-graphics-maintainer@vmware.com >> Cc: Thomas Hellstrom thellstrom@vmware.com >> --- >> 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.
/Thomas