KY Srinivasan, On Mon, Dec 15, 2014 at 07:00:45AM +0000, KY Srinivasan wrote: > > > > -----Original Message----- > > From: Jeremiah Mahler [mailto:jmmah...@gmail.com] > > Sent: Wednesday, December 10, 2014 6:10 PM > > To: KY Srinivasan > > Cc: gre...@linuxfoundation.org; linux-kernel@vger.kernel.org; > > de...@linuxdriverproject.org; o...@aepfle.de; a...@canonical.com; > > jasow...@redhat.com; mc...@ipxe.org > > Subject: Re: [PATCH V3 1/1] Drivers: hv: vmbus: Fix a bug in > > vmbus_establish_gpadl() > > > > K. Y. Srinivasan, > > > > On Wed, Dec 10, 2014 at 05:13:00PM -0800, K. Y. Srinivasan wrote: > > > Correctly compute the local (gpadl) handle. > > > > This description is still too sparse for me. How was it computed before and > > why was this incorrect? Pretend like you are trying to explain your patch > > to > > someone who has no idea what you did. > > > > > I would like to thank Michael Brown <mc...@ipxe.org> for seeing this bug. > > > > > > Signed-off-by: K. Y. Srinivasan <k...@microsoft.com> > > > Reported-by: Michael Brown <mc...@ipxe.org> > > > --- > > > Changes in V2: Added the Reported-by tag. > > > Changes in V3: Cleaned up the commit log. > > > > > > drivers/hv/channel.c | 4 ++-- > > > 1 files changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c index > > > 433f72a..c76ffbe 100644 > > > --- a/drivers/hv/channel.c > > > +++ b/drivers/hv/channel.c > > > @@ -366,8 +366,8 @@ int vmbus_establish_gpadl(struct vmbus_channel > > *channel, void *kbuffer, > > > unsigned long flags; > > > int ret = 0; > > > > > > - next_gpadl_handle = > > atomic_read(&vmbus_connection.next_gpadl_handle); > > > - atomic_inc(&vmbus_connection.next_gpadl_handle); > > > + next_gpadl_handle = > > > + > > (atomic_inc_return(&vmbus_connection.next_gpadl_handle) - 1); > > > > > Tell me if I understand this correctly. > > > > Before it read the handle and incremented it. > > > > y = x + 1 > > > > Now it reads the handle, increments it, then decrements it. > > > > y = (x + 1) - 1 = x > > This code can be executed concurrently on multiple CPUs. We want to ensure > that each call to > establish gpadl gets a unique local handle. The earlier code was buggy in > that we would read the > handle and then atomically increment it. Thus, multiple CPUs could read the > identical current > value which would be their local handle. What we want is the ability to > atomically read and increment > the value - this would ensure that each caller got a unique value even if > they executed the code > concurrently on multiple CPUs. The API atomic_inc_return(), atomically > increments and returns the > incremented value. We locally decrement this value to emulate the logic of > "read the current value and > atomically increment the value. > > Hope this helps, > > K. Y > > [...]
So to avoid concurrency issues you used a single atomic operation instead of two separate operations. That make sense. But it still doesn't explain why you changed the calculation by subtracting 1. -- - Jeremiah Mahler -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
