Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

Am 24.08.2018 um 13:52 schrieb Michal Hocko:
> On Fri 24-08-18 13:43:16, Christian König wrote:
> > Am 24.08.2018 um 13:32 schrieb Michal Hocko:
> > > On Fri 24-08-18 19:54:19, Tetsuo Handa wrote:
> > > > Two more worries for this patch.
> > > >
> > > >
> > > >
> > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
> > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mn.c
> > > > > @@ -178,12 +178,18 @@ void amdgpu_mn_unlock(struct amdgpu_mn *mn)
> > > > >     *
> > > > >     * @amn: our notifier
> > > > >     */
> > > > > -static void amdgpu_mn_read_lock(struct amdgpu_mn *amn)
> > > > > +static int amdgpu_mn_read_lock(struct amdgpu_mn *amn, bool blockable)
> > > > >    {
> > > > > -       mutex_lock(&amn->read_lock);
> > > > > +       if (blockable)
> > > > > +               mutex_lock(&amn->read_lock);
> > > > > +       else if (!mutex_trylock(&amn->read_lock))
> > > > > +               return -EAGAIN;
> > > > > +
> > > > >           if (atomic_inc_return(&amn->recursion) == 1)
> > > > >                   down_read_non_owner(&amn->lock);
> > > > Why don't we need to use trylock here if blockable == false ?
> > > > Want comment why it is safe to use blocking lock here.
> > > Hmm, I am pretty sure I have checked the code but it was quite confusing
> > > so I might have missed something. Double checking now, it seems that
> > > this read_lock is not used anywhere else and it is not _the_ lock we are
> > > interested about. It is the amn->lock (amdgpu_mn_lock) which matters as
> > > it is taken in exclusive mode for expensive operations.
> > The write side of the lock is only taken in the command submission IOCTL.
> >
> > So you actually don't need to change anything here (even the proposed
> > changes are overkill) since we can't tear down the struct_mm while an IOCTL
> > is still using.
> I am not so sure. We are not in the mm destruction phase yet. This is
> mostly about the oom context which might fire right during the IOCTL. If
> any of the path which is holding the write lock blocks for unbound
> amount of time or even worse allocates a memory then we are screwed. So
> we need to back of when blockable = false.
Oh, yeah good point. Haven't thought about that possibility.

> > > Is that correct Christian? If this is correct then we need to update the
> > > locking here. I am struggling to grasp the ref counting part. Why cannot
> > > all readers simply take the lock rather than rely on somebody else to
> > > take it? 1ed3d2567c800 didn't really help me to understand the locking
> > > scheme here so any help would be appreciated.
> > That won't work like this there might be multiple
> > invalidate_range_start()/invalidate_range_end() pairs open at the same time.
> > E.g. the lock might be taken recursively and that is illegal for a
> > rw_semaphore.
> I am not sure I follow. Are you saying that one invalidate_range might
> trigger another one from the same path?
No, but what can happen is:

invalidate_range_start(A,B);
invalidate_range_start(C,D);
...
invalidate_range_end(C,D);
invalidate_range_end(A,B);

Grabbing the read lock twice would be illegal in this case.

Regards,
Christian.

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