On Tue, Jul 12, 2016 at 01:14:41PM +0100, Chris Wilson wrote: > 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.) If we change the kernel to just merge everything together when importing a new fence I think it should be perfectly save. I.e. 1) grab reservation lock 2) assemble a fence_array with the current fences + the new one passed in through sync_file. 3) put that into the right slot 4) unlock If we switch reservations over to fence_array it might even be somewhat pretty. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
