Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns the fence

[Akhil P Oommen <akhilpo@codeaurora.org>]

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On 6/25/2018 1:20 PM, Daniel Vetter wrote:
> On Fri, Jun 22, 2018 at 11:08:48AM +0100, Chris Wilson wrote:
>> Quoting Gustavo Padovan (2018-06-22 11:04:16)
>>> Hi Akhil,
>>>
>>> On Fri, 2018-06-22 at 15:10 +0530, Akhil P Oommen wrote:
>>>> Each fence object holds function pointers of the module that
>>>> initialized
>>>> it. Allowing the module to unload before this fence's release is
>>>> catastrophic. So, keep a refcount on the module until the fence is
>>>> released.
>>>>
>>>> Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
>>>> ---
>>>> Changes in v2:
>>>> - added description for the new function parameter.
>>>>
>>>>   drivers/dma-buf/dma-fence.c | 16 +++++++++++++---
>>>>   include/linux/dma-fence.h   | 10 ++++++++--
>>>>   2 files changed, 21 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-
>>>> fence.c
>>>> index 4edb9fd..2aaa44e 100644
>>>> --- a/drivers/dma-buf/dma-fence.c
>>>> +++ b/drivers/dma-buf/dma-fence.c
>>>> @@ -18,6 +18,7 @@
>>>>    * more details.
>>>>    */
>>>>   
>>>> +#include <linux/module.h>
>>>>   #include <linux/slab.h>
>>>>   #include <linux/export.h>
>>>>   #include <linux/atomic.h>
>>>> @@ -168,6 +169,7 @@ void dma_fence_release(struct kref *kref)
>>>>   {
>>>>        struct dma_fence *fence =
>>>>                container_of(kref, struct dma_fence, refcount);
>>>> +     struct module *module = fence->owner;
>>>>   
>>>>        trace_dma_fence_destroy(fence);
>>>>   
>>>> @@ -178,6 +180,8 @@ void dma_fence_release(struct kref *kref)
>>>>                fence->ops->release(fence);
>>>>        else
>>>>                dma_fence_free(fence);
>>>> +
>>>> +     module_put(module);
>>>>   }
>>>>   EXPORT_SYMBOL(dma_fence_release);
>>>>   
>>>> @@ -541,6 +545,7 @@ struct default_wait_cb {
>>>>   
>>>>   /**
>>>>    * dma_fence_init - Initialize a custom fence.
>>>> + * @module:  [in]    the module that calls this API
>>>>    * @fence:   [in]    the fence to initialize
>>>>    * @ops:     [in]    the dma_fence_ops for operations on this
>>>> fence
>>>>    * @lock:    [in]    the irqsafe spinlock to use for locking
>>>> this fence
>>>> @@ -556,8 +561,9 @@ struct default_wait_cb {
>>>>    * to check which fence is later by simply using dma_fence_later.
>>>>    */
>>>>   void
>>>> -dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops
>>>> *ops,
>>>> -            spinlock_t *lock, u64 context, unsigned seqno)
>>>> +_dma_fence_init(struct module *module, struct dma_fence *fence,
>>>> +             const struct dma_fence_ops *ops, spinlock_t *lock,
>>>> +             u64 context, unsigned seqno)
>>>>   {
>>>>        BUG_ON(!lock);
>>>>        BUG_ON(!ops || !ops->wait || !ops->enable_signaling ||
>>>> @@ -571,7 +577,11 @@ struct default_wait_cb {
>>>>        fence->seqno = seqno;
>>>>        fence->flags = 0UL;
>>>>        fence->error = 0;
>>>> +     fence->owner = module;
>>>> +
>>>> +     if (!try_module_get(module))
>>>> +             fence->owner = NULL;
>>>>   
>>>>        trace_dma_fence_init(fence);
>>>>   }
>>>> -EXPORT_SYMBOL(dma_fence_init);
>>>> +EXPORT_SYMBOL(_dma_fence_init);
>>> Do we still need to export the symbol, it won't be called from outside
>>> anymore? Other than that looks good to me:
>> There's a big drawback in that a module reference is often insufficient,
>> and that a reference on the driver (or whatever is required for the
>> lifetime of the fence) will already hold the module reference.
>>
>> Considering that we want a few 100k fences in flight per second, is
>> there no other way to only export a fence with a module reference?
> We'd need to make the timeline a full-blown object (Maarten owes me one
> for that design screw-up), and then we could stuff all these things in
> there.
>
> And I think that's the right fix, since try_module_get for every
> dma_fence_init just ain't cool really :-)
> -Daniel
Thanks for the feedback, Daniel.
I see your point, but I am not sure how much impact an extra refcounting 
would create considering the whole effort of setting up a new fence. 
Also, this refcounting is not required for built-in modules.

As of now, unloading a kernel module that uses fence_init() is an easy 
way to bring down the system. This patch simply fixes that. What you 
have suggested sounds like a non-trivial effort which someone who is more
familiar with this code base can do a better job than me. Perhaps we can 
take this patch now to fix the issue at hand and later somebody else can 
share a more optimal solution. :)

@Gustavo & @Sumit, I would like the maintainers to take a decision here.

-Akhil.

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