[PATCH] dma-buf: fix and rework dma_buf_poll v3
Christian König
ckoenig.leichtzumerken at gmail.com
Wed Jun 23 11:17:02 UTC 2021
Am 22.06.21 um 19:02 schrieb Daniel Vetter:
> On Tue, Jun 22, 2021 at 3:07 PM Christian König
> <ckoenig.leichtzumerken at gmail.com> wrote:
>> Crap, hit enter to early before adding a cover letter.
>>
>> This is the same patch as before, but as requested I'm keeping the
>> exclusive fence handling as it is for now.
>>
>> Daniel can you double check this and/or make sure that it is tested?
>>
>> I only smoke tested it and the code is so complicated that I'm not sure
>> I catched all side effects.
> So I've thought about this some more, and we actually have docs for
> how this is supposed to work:
>
> https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html#implicit-fence-poll-support
>
> Docs are pretty clear that we want both read and write for EPOLLOUT or
> well both exclusive and shared fences. So I guess back to your actual
> thing, but maybe we should get some acks from userspace people for it
> (Michel, Pekka, Simon probably usual suspects).
Ok, sounds good to me. Going to send out a patch rebased to
drm-misc-fixes today.
>
> The other thing I brought up and I haven't seen you reply to (maybe
> missed it) is whether we shouldn't just use dma_resv_get_fences(). We
> need to do the refcounting anyway, and this avoids us having to
> open-code this very nasty code. Finally, and imo most important, this
> is what's also used in drm_gem_fence_array_add_implicit(), which many
> drivers use to handle their implicit in-fences. So would be nice to
> have exactly matching code between that and what dma-buf poll does for
> "can I read" and "can I write".
>
> Thoughts?
Yeah, I've seen that. Just didn't had time to answer.
That goes into the same direction as my thinking that we need to
centralize the RCU and synchronization handling in general.
What I don't like about the approach is that dma_resv_get_fences() needs
to allocate memory. For a lot of use cases including dma_buf_poll that
is rather overkill.
To unify the handling I think we should use the iterator I've create the
prototype of. This way we only have an "for_write" parameter and the
iterator in return gives you all the fences you need.
When you then extend that approach we could say we have an enum
describing the use case. Something like:
1. For explicit sync, just give me all the must sync fences from memory
management.
2. For read, give me all the writers and memory management fences.
3. For write, give me all the readers and writers and memory management
fences.
4. For memory management, give me everything including things like PTE
updates/TLB flushes.
So instead of asking for some specific type of fence(s) the drivers
tells the dma_resv object about their use case and in return get the
fences they need to wait for.
This essentially means that we move the decision what to wait for from
the drivers into the dma_resv object, which I think would be a massive
improvement.
Functions like dma_resv_get_list(), dma_resv_get_excl(),
dma_resv_get_excl_rcu() etc would then essentially be deprecated and
instead you use dma_resv_get_fences(), dma_resv_for_each_fences(),
dma_resv_wait_timeout(), dma_resv_test_signaled() with a proper use case.
What do you think?
Christian.
> -Daniel
>
>> Regards,
>> Christian.
>>
>> Am 22.06.21 um 15:04 schrieb Christian König:
>>> Daniel pointed me towards this function and there are multiple obvious problems
>>> in the implementation.
>>>
>>> First of all the retry loop is not working as intended. In general the retry
>>> makes only sense if you grab the reference first and then check the sequence
>>> values.
>>>
>>> It's also good practice to keep the reference around when installing callbacks
>>> to fences you don't own.
>>>
>>> And last the whole implementation was unnecessary complex and rather hard to
>>> understand which could lead to probably unexpected behavior of the IOCTL.
>>>
>>> Fix all this by reworking the implementation from scratch.
>>>
>>> Only mildly tested and needs a thoughtful review of the code.
>>>
>>> v2: fix the reference counting as well
>>> v3: keep the excl fence handling as is for stable
>>>
>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>>> CC: stable at vger.kernel.org
>>> ---
>>> drivers/dma-buf/dma-buf.c | 133 ++++++++++++++++----------------------
>>> include/linux/dma-buf.h | 2 +-
>>> 2 files changed, 55 insertions(+), 80 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index eadd1eaa2fb5..e97c3a9d98d5 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -72,7 +72,7 @@ static void dma_buf_release(struct dentry *dentry)
>>> * If you hit this BUG() it means someone dropped their ref to the
>>> * dma-buf while still having pending operation to the buffer.
>>> */
>>> - BUG_ON(dmabuf->cb_shared.active || dmabuf->cb_excl.active);
>>> + BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);
>>>
>>> dmabuf->ops->release(dmabuf);
>>>
>>> @@ -202,16 +202,20 @@ static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
>>> wake_up_locked_poll(dcb->poll, dcb->active);
>>> dcb->active = 0;
>>> spin_unlock_irqrestore(&dcb->poll->lock, flags);
>>> + dma_fence_put(fence);
>>> }
>>>
>>> static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>>> {
>>> + struct dma_buf_poll_cb_t *dcb;
>>> struct dma_buf *dmabuf;
>>> struct dma_resv *resv;
>>> struct dma_resv_list *fobj;
>>> struct dma_fence *fence_excl;
>>> - __poll_t events;
>>> unsigned shared_count, seq;
>>> + struct dma_fence *fence;
>>> + __poll_t events;
>>> + int r, i;
>>>
>>> dmabuf = file->private_data;
>>> if (!dmabuf || !dmabuf->resv)
>>> @@ -225,99 +229,70 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>>> if (!events)
>>> return 0;
>>>
>>> + dcb = events & EPOLLOUT ? &dmabuf->cb_out : &dmabuf->cb_in;
>>> +
>>> + /* Only queue a new one if we are not still waiting for the old one */
>>> + spin_lock_irq(&dmabuf->poll.lock);
>>> + if (dcb->active)
>>> + events = 0;
>>> + else
>>> + dcb->active = events;
>>> + spin_unlock_irq(&dmabuf->poll.lock);
>>> + if (!events)
>>> + return 0;
>>> +
>>> retry:
>>> seq = read_seqcount_begin(&resv->seq);
>>> rcu_read_lock();
>>>
>>> fobj = rcu_dereference(resv->fence);
>>> - if (fobj)
>>> + if (fobj && events & EPOLLOUT)
>>> shared_count = fobj->shared_count;
>>> else
>>> shared_count = 0;
>>> - fence_excl = rcu_dereference(resv->fence_excl);
>>> - if (read_seqcount_retry(&resv->seq, seq)) {
>>> - rcu_read_unlock();
>>> - goto retry;
>>> - }
>>>
>>> - if (fence_excl && (!(events & EPOLLOUT) || shared_count == 0)) {
>>> - struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_excl;
>>> - __poll_t pevents = EPOLLIN;
>>> -
>>> - if (shared_count == 0)
>>> - pevents |= EPOLLOUT;
>>> -
>>> - spin_lock_irq(&dmabuf->poll.lock);
>>> - if (dcb->active) {
>>> - dcb->active |= pevents;
>>> - events &= ~pevents;
>>> - } else
>>> - dcb->active = pevents;
>>> - spin_unlock_irq(&dmabuf->poll.lock);
>>> -
>>> - if (events & pevents) {
>>> - if (!dma_fence_get_rcu(fence_excl)) {
>>> - /* force a recheck */
>>> - events &= ~pevents;
>>> - dma_buf_poll_cb(NULL, &dcb->cb);
>>> - } else if (!dma_fence_add_callback(fence_excl, &dcb->cb,
>>> - dma_buf_poll_cb)) {
>>> - events &= ~pevents;
>>> - dma_fence_put(fence_excl);
>>> - } else {
>>> - /*
>>> - * No callback queued, wake up any additional
>>> - * waiters.
>>> - */
>>> - dma_fence_put(fence_excl);
>>> - dma_buf_poll_cb(NULL, &dcb->cb);
>>> - }
>>> + for (i = 0; i < shared_count; ++i) {
>>> + fence = rcu_dereference(fobj->shared[i]);
>>> + fence = dma_fence_get_rcu(fence);
>>> + if (!fence || read_seqcount_retry(&resv->seq, seq)) {
>>> + /* Concurrent modify detected, force re-check */
>>> + dma_fence_put(fence);
>>> + rcu_read_unlock();
>>> + goto retry;
>>> }
>>> - }
>>>
>>> - if ((events & EPOLLOUT) && shared_count > 0) {
>>> - struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_shared;
>>> - int i;
>>> -
>>> - /* Only queue a new callback if no event has fired yet */
>>> - spin_lock_irq(&dmabuf->poll.lock);
>>> - if (dcb->active)
>>> - events &= ~EPOLLOUT;
>>> - else
>>> - dcb->active = EPOLLOUT;
>>> - spin_unlock_irq(&dmabuf->poll.lock);
>>> -
>>> - if (!(events & EPOLLOUT))
>>> + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
>>> + if (!r) {
>>> + /* Callback queued */
>>> + events = 0;
>>> goto out;
>>> + }
>>> + dma_fence_put(fence);
>>> + }
>>>
>>> - for (i = 0; i < shared_count; ++i) {
>>> - struct dma_fence *fence = rcu_dereference(fobj->shared[i]);
>>> -
>>> - if (!dma_fence_get_rcu(fence)) {
>>> - /*
>>> - * fence refcount dropped to zero, this means
>>> - * that fobj has been freed
>>> - *
>>> - * call dma_buf_poll_cb and force a recheck!
>>> - */
>>> - events &= ~EPOLLOUT;
>>> - dma_buf_poll_cb(NULL, &dcb->cb);
>>> - break;
>>> - }
>>> - if (!dma_fence_add_callback(fence, &dcb->cb,
>>> - dma_buf_poll_cb)) {
>>> - dma_fence_put(fence);
>>> - events &= ~EPOLLOUT;
>>> - break;
>>> - }
>>> + fence = dma_resv_get_excl(resv);
>>> + if (fence && shared_count == 0) {
>>> + fence = dma_fence_get_rcu(fence);
>>> + if (!fence || read_seqcount_retry(&resv->seq, seq)) {
>>> + /* Concurrent modify detected, force re-check */
>>> dma_fence_put(fence);
>>> + rcu_read_unlock();
>>> + goto retry;
>>> +
>>> }
>>>
>>> - /* No callback queued, wake up any additional waiters. */
>>> - if (i == shared_count)
>>> - dma_buf_poll_cb(NULL, &dcb->cb);
>>> + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
>>> + if (!r) {
>>> + /* Callback queued */
>>> + events = 0;
>>> + goto out;
>>> + }
>>> + dma_fence_put(fence_excl);
>>> }
>>>
>>> + /* No callback queued, wake up any additional waiters. */
>>> + dma_buf_poll_cb(NULL, &dcb->cb);
>>> +
>>> out:
>>> rcu_read_unlock();
>>> return events;
>>> @@ -562,8 +537,8 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
>>> dmabuf->owner = exp_info->owner;
>>> spin_lock_init(&dmabuf->name_lock);
>>> init_waitqueue_head(&dmabuf->poll);
>>> - dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll;
>>> - dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0;
>>> + dmabuf->cb_in.poll = dmabuf->cb_out.poll = &dmabuf->poll;
>>> + dmabuf->cb_in.active = dmabuf->cb_out.active = 0;
>>>
>>> if (!resv) {
>>> resv = (struct dma_resv *)&dmabuf[1];
>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>>> index efdc56b9d95f..7e747ad54c81 100644
>>> --- a/include/linux/dma-buf.h
>>> +++ b/include/linux/dma-buf.h
>>> @@ -329,7 +329,7 @@ struct dma_buf {
>>> wait_queue_head_t *poll;
>>>
>>> __poll_t active;
>>> - } cb_excl, cb_shared;
>>> + } cb_in, cb_out;
>>> };
>>>
>>> /**
>
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