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Subject: Re: [PATCH v2] dma-buf/fence: Take refcount on the module that owns
 the fence
To:     Chris Wilson <chris@chris-wilson.co.uk>,
        Gustavo Padovan <gustavo@padovan.org>, sumit.semwal@linaro.org,
        jcrouse@codeaurora.org, linux-media@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
        linux-kernel@vger.kernel.org
Cc:     linux-arm-msm@vger.kernel.org, smasetty@codeaurora.org
References: <1529660407-6266-1-git-send-email-akhilpo@codeaurora.org>
 <1529661856.7034.404.camel@padovan.org>
 <152966212844.11773.6596589902326100250@mail.alporthouse.com>
From:   Akhil P Oommen <akhilpo@codeaurora.org>
Message-ID: <cb950fea-b0cc-bbe7-9e94-78c62849cb64@codeaurora.org>
Date:   Fri, 22 Jun 2018 17:12:05 +0530
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On 6/22/2018 3:38 PM, 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:
Yes. Because dma_fence_init() is now a macro that resolves to 
_dma_fence_init().
> 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.
I didn't quite understand what you meant here. Could you please elaborate?
>
> 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?
> -Chris

Thanks,
Akhil.