> Am 24.09.2020 um 23:26 schrieb Dan Williams <dan.j.williams@xxxxxxxxx>: > > [..] >>> I'm not suggesting to busy the whole "virtio" range, just the portion >>> that's about to be passed to add_memory_driver_managed(). >> >> I'm afraid I don't get your point. For virtio-mem: >> >> Before: >> >> 1. Create virtio0 container resource >> >> 2. (somewhen in the future) add_memory_driver_managed() >> - Create resource (System RAM (virtio_mem)), marking it busy/driver >> managed >> >> After: >> >> 1. Create virtio0 container resource >> >> 2. (somewhen in the future) Create resource (System RAM (virtio_mem)), >> marking it busy/driver managed >> 3. add_memory_driver_managed() >> >> Not helpful or simpler IMHO. > > The concern I'm trying to address is the theoretical race window and > layering violation in this sequence in the kmem driver: > > 1/ res = request_mem_region(...); > 2/ res->flags = IORESOURCE_MEM; > 3/ add_memory_driver_managed(); > > Between 2/ and 3/ something can race and think that it owns the > region. Do I think it will happen in practice, no, but it's still a > pattern that deserves come cleanup. I think in that unlikely event (rather impossible), add_memory_driver_managed() should fail, detecting a conflicting (busy) resource. Not sure what will happen next ( and did not double-check). But yeah, the way the BUSY bit is cleared here is wrong - simply overwriting other bits. And it would be even better if we could avoid manually messing with flags here. >
