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@xxxxxxxxx] > > > Sent: Wednesday, December 10, 2014 6:10 PM > > > To: KY Srinivasan > > > Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > > > devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; > > > jasowang@xxxxxxxxxx; mcb30@xxxxxxxx > > > 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@xxxxxxxx> for seeing this bug. > > > > > > > > Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx> > > > > Reported-by: Michael Brown <mcb30@xxxxxxxx> > > > > --- > > > > 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 _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
