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    SubjectRe: [PATCH V3 1/1] Drivers: hv: vmbus: Fix a bug in vmbus_establish_gpadl()
    On Sun, Dec 14, 2014 at 11:59:19PM -0800, Jeremiah Mahler wrote:
    > KY Srinivasan,
    >
    > On Mon, Dec 15, 2014 at 07:00:45AM +0000, KY Srinivasan wrote:
    > >
    > >
    > > > -----Original Message-----
    > > > From: Jeremiah Mahler [mailto:jmmahler@gmail.com]
    > > > Sent: Wednesday, December 10, 2014 6:10 PM
    > > > To: KY Srinivasan
    > > > Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
    > > > devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
    > > > jasowang@redhat.com; mcb30@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 <mcb30@ipxe.org> for seeing this bug.
    > > > >
    > > > > Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
    > > > > Reported-by: Michael Brown <mcb30@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.

    The calculation appears identical to my reading, the original form was:

    next_gpadl_handle = atomic_read(&vmbus_connection.next_gpadl_handle);
    atomic_inc(&vmbus_connection.next_gpadl_handle);

    or:

    y = x;
    x++;

    so y == x' (x before incrementing)

    the new code is:

    next_gpadl_handle = (atomic_inc_return(&vmbus_connection.next_gpadl_handle) - 1);

    or:

    y = ++x - 1;

    Also making y = x' (x before incrementing)

    -apw


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