On Sun, May 31, 2009 at 03:14:36PM +0300, Avi Kivity wrote: > Marcelo Tosatti wrote: >> Move coalesced_mmio locking to its own device, instead of relying on >> kvm->lock. >> >> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> >> >> Index: kvm-irqlock/virt/kvm/coalesced_mmio.c >> =================================================================== >> --- kvm-irqlock.orig/virt/kvm/coalesced_mmio.c >> +++ kvm-irqlock/virt/kvm/coalesced_mmio.c >> @@ -26,9 +26,7 @@ static int coalesced_mmio_in_range(struc >> if (!is_write) >> return 0; >> - /* kvm->lock is taken by the caller and must be not released before >> - * dev.read/write >> - */ >> + spin_lock(&dev->lock); >> > > This unbalanced locking is still very displeasing. At a minimum you > need a sparse annotation to indicate it. > > But I think it really indicates a problem with the io_device API. > > Potential solutions: > - fold in_range() into ->write and ->read. Make those functions > responsible for both determining whether they can handle the range and > performing the I/O. > - have a separate rwlock for the device list. IMO the problem is the coalesced_mmio device. The unbalanced locking is a result of the abuse of the in_range() and read/write() methods. Normally you'd expect parallel accesses to in_range() to be allowed, since its just checking whether (aha) the access is in range, returning a pointer to the device if positive. Now read/write() are the ones who need serialization, since they touch the device internal state. coalesced_mmio abuses in_range() to do more things than it should. Ideally we should fix coalesced_mmio, but i'm not going to do that now (sorry, not confident in changing it without seeing go through intense torture testing). That said, is sparse annotation enough the convince you? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
