linux-kernel - Re: [PATCH 1/1] drm/amdkfd: Protect the Client whilst it is being operated on

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Message-ID: <YjsWvy8cT2eOw618@google.com>
Date:   Wed, 23 Mar 2022 12:46:55 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     philip yang <yangp@....com>
Cc:     Felix Kuehling <felix.kuehling@....com>,
        David Airlie <airlied@...ux.ie>,
        "Pan, Xinhui" <Xinhui.Pan@....com>, linux-kernel@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH 1/1] drm/amdkfd: Protect the Client whilst it is being
 operated on

On Thu, 17 Mar 2022, Lee Jones wrote:

> On Thu, 17 Mar 2022, philip yang wrote:
> 
> >    On 2022-03-17 11:13 a.m., Lee Jones wrote:
> > 
> > On Thu, 17 Mar 2022, Felix Kuehling wrote:
> > 
> > 
> > Am 2022-03-17 um 11:00 schrieb Lee Jones:
> > 
> > Good afternoon Felix,
> > 
> > Thanks for your review.
> > 
> > 
> > Am 2022-03-17 um 09:16 schrieb Lee Jones:
> > 
> > Presently the Client can be freed whilst still in use.
> > 
> > Use the already provided lock to prevent this.
> > 
> > Cc: Felix Kuehling [1]<Felix.Kuehling@....com>
> > Cc: Alex Deucher [2]<alexander.deucher@....com>
> > Cc: "Christian König" [3]<christian.koenig@....com>
> > Cc: "Pan, Xinhui" [4]<Xinhui.Pan@....com>
> > Cc: David Airlie [5]<airlied@...ux.ie>
> > Cc: Daniel Vetter [6]<daniel@...ll.ch>
> > Cc: [7]amd-gfx@...ts.freedesktop.org
> > Cc: [8]dri-devel@...ts.freedesktop.org
> > Signed-off-by: Lee Jones [9]<lee.jones@...aro.org>
> > ---
> >    drivers/gpu/drm/amd/amdkfd/kfd_smi_events.c | 6 ++++++
> >    1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_smi_events.c b/drivers/gpu/drm/amd/a
> > mdkfd/kfd_smi_events.c
> > index e4beebb1c80a2..3b9ac1e87231f 100644
> > --- a/drivers/gpu/drm/amd/amdkfd/kfd_smi_events.c
> > +++ b/drivers/gpu/drm/amd/amdkfd/kfd_smi_events.c
> > @@ -145,8 +145,11 @@ static int kfd_smi_ev_release(struct inode *inode, struct f
> > ile *filep)
> >         spin_unlock(&dev->smi_lock);
> >         synchronize_rcu();
> > +
> > +       spin_lock(&client->lock);
> >         kfifo_free(&client->fifo);
> >         kfree(client);
> > +       spin_unlock(&client->lock);
> > 
> > The spin_unlock is after the spinlock data structure has been freed.
> > 
> > Good point.
> > 
> > If we go forward with this approach the unlock should perhaps be moved
> > to just before the kfree().
> > 
> > 
> > There
> > should be no concurrent users here, since we are freeing the data structure.
> > If there still are concurrent users at this point, they will crash anyway.
> > So the locking is unnecessary.
> > 
> > The users may well crash, as does the kernel unfortunately.
> > 
> > We only get to kfd_smi_ev_release when the file descriptor is closed. User
> > mode has no way to use the client any more at this point. This function also
> > removes the client from the dev->smi_cllients list. So no more events will
> > be added to the client. Therefore it is safe to free the client.
> > 
> > If any of the above were not true, it would not be safe to kfree(client).
> > 
> > But if it is safe to kfree(client), then there is no need for the locking.
> > 
> > I'm not keen to go into too much detail until it's been patched.
> > 
> > However, there is a way to free the client while it is still in use.
> > 
> > Remember we are multi-threaded.
> > 
> >    files_struct->count refcount is used to handle this race, as
> >    vfs_read/vfs_write takes file refcount and fput calls release only if
> >    refcount is 1, to guarantee that read/write from user space is finished
> >    here.
> > 
> >    Another race is driver add_event_to_kfifo while closing the handler. We
> >    use rcu_read_lock in add_event_to_kfifo, and kfd_smi_ev_release calls
> >    synchronize_rcu to wait for all rcu_read done. So it is safe to call
> >    kfifo_free(&client->fifo) and kfree(client).
> 
> Philip, please reach out to Felix.

Philip, Felix, are you receiving my direct messages?

I have a feeling they're being filtered out by AMD's mail server.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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