On Wed, Sep 09, 2015 at 03:45:40PM +0100, Tvrtko Ursulin wrote: > On 08/10/2015 09:51 AM, Chris Wilson wrote: > >Whilst discussing possible ways to trigger an invalidate_range on a > >userptr with an aliased GGTT mmapping (and so cause a struct_mutex > >deadlock), the conclusion is that we can, and we must, prevent any > >possible deadlock by avoiding taking the mutex at all during > >invalidate_range. This has numerous advantages all of which stem from > >avoid the sleeping function from inside the unknown context. In > >particular, it simplifies the invalidate_range because we no longer > >have to juggle the spinlock/mutex and can just hold the spinlock > >for the entire walk. To compensate, we have to make get_pages a bit more > >complicated in order to serialise with a pending cancel_userptr worker. > >As we hold the struct_mutex, we have no choice but to return EAGAIN and > >hope that the worker is then flushed before we retry after reacquiring > >the struct_mutex. > > > >The important caveat is that the invalidate_range itself is no longer > >synchronous. There exists a small but definite period in time in which > >the old PTE's page remain accessible via the GPU. Note however that the > >physical pages themselves are not invalidated by the mmu_notifier, just > >the CPU view of the address space. The impact should be limited to a > >delay in pages being flushed, rather than a possibility of writing to > >the wrong pages. The only race condition that this worsens is remapping > >an userptr active on the GPU where fresh work may still reference the > >old pages due to struct_mutex contention. Given that userspace is racing > >with the GPU, it is fair to say that the results are undefined. > > > >v2: Only queue (and importantly only take one refcnt) the worker once. > > This one I looked at at the time of previous posting and it looked > fine, minus one wrong line of thinking of mine. On a brief look it > still looks good, so: > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> > > I assume Michał has run all these through the relevant test cases? > > Slightly related, I now worry about the WARN_ONs in > __cancel_userptr__worker since they look to be triggerable by > malicious userspace which is not good. They could always be I thought, if you could somehow pin the userptr into a hardware register and then unmap the vma. That is a scary thought and one I would like a WARN for. That should be the only way, and I shudder at the prospect of working out who to send the SIGBUS to. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
