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