On Tue, Jul 12, 2016 at 01:53:56PM +0200, Christian König wrote: > Am 11.07.2016 um 23:59 schrieb Chris Wilson: > >When dealing with user interfaces that utilize explicit fences, it is > >convenient to sometimes create those fences after the fact, i.e. to > >query the dma-buf for the current set of implicit fences, encapsulate > >those into a sync_file and hand that fd back to userspace. > >Correspondingly, being able to add an explicit fence back into the mix > >of fences being tracked by the dma-buf allows that userspace fence to be > >included in any implicit tracking. > > Well I think that this is a rather questionable interface. > > For example how do you deal with race conditions? E.g. between > querying the fences from the reservation object and adding a new one > we could gain new fences because of the kernel swapping things out > or another thread making some submission with this buffer. > > Additional to that IIRC reservation_object_add_excl_fence() > currently replaces all shared fences with the exclusive one. A > malicious application could use this to trick the kernel driver into > thinking that this buffer object is idle while it is still accessed > by some operation. At least with GPU operations you can easily take > over the system when you manage to get access to a page table with > this.
The only difference here is that we believe the GPU drivers to enforce the ordering between each other. So we can either insert a wait before adding the exclusive fence, or we can just not expose an import ioctl. Extracting the set of fences isn't an issue? (That's the part that has a more legitimate usecase.) -Chris -- Chris Wilson, Intel Open Source Technology Centre
