On Mon, Jun 25, 2018 at 09:21:15PM +0530, Akhil P Oommen wrote:

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. :)

Module unload is a developer-only feature for a reason. Given that I don't think fixing this with a hack is the right approach. And dma_fence_init() is supposed to be really fast.

Also note that you can fix this already for your own driver by simply waiting for all pending dma_fences to get released, so I don't think it's super-important to land this asap.

Yes the real fix is a bit more involved, but shouldn't be too hard to pull off really. -Daniel

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

-Akhil.

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