Hi,
Currently, there is no read barrier between reading the index
(kfifo.in) and fetching the real data from the fifo.
I am afraid that will cause the vfifo is observed as not empty
however the data is not actually ready for read. Right?
Thanks!
My guts thinks you are right. Feel free to send a patch...
Am Freitag, den 11.05.2018, 15:25 +0800 schrieb Xiao Guangrong:
> Hi,
>
> Currently, there is no read barrier between reading the index
> (kfifo.in) and fetching the real data from the fifo.
>
> I am afraid that will cause the vfifo is observed as not empty
> however the data is not actually ready for read. Right?
>
> Thanks!
On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote:
>
> Hi,
>
> Currently, there is no read barrier between reading the index
> (kfifo.in) and fetching the real data from the fifo.
>
> I am afraid that will cause the vfifo is observed as not empty
> however the data is not actually ready for read. Right?
That code is decidedly dodgy indeed. I can only see smp_wmb() but no
matching barriers at all -- therefore the code is almost certainly as
good as not having any barriers at all.
I would suggest you try and convert the code to smp_store_release() and
smp_load_acquire() while you're at it.
On Fri, May 11, 2018 at 10:32:42AM +0200, Peter Zijlstra wrote:
> On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote:
> >
> > Hi,
> >
> > Currently, there is no read barrier between reading the index
> > (kfifo.in) and fetching the real data from the fifo.
> >
> > I am afraid that will cause the vfifo is observed as not empty
> > however the data is not actually ready for read. Right?
>
> That code is decidedly dodgy indeed. I can only see smp_wmb() but no
> matching barriers at all -- therefore the code is almost certainly as
> good as not having any barriers at all.
>
> I would suggest you try and convert the code to smp_store_release() and
> smp_load_acquire() while you're at it.
Isn't this one of the places where we rely on control dependencies?
Thanx, Paul
On Fri, May 11, 2018 at 09:20:53AM -0700, Paul E. McKenney wrote:
> On Fri, May 11, 2018 at 10:32:42AM +0200, Peter Zijlstra wrote:
> > On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote:
> > >
> > > Hi,
> > >
> > > Currently, there is no read barrier between reading the index
> > > (kfifo.in) and fetching the real data from the fifo.
> > >
> > > I am afraid that will cause the vfifo is observed as not empty
> > > however the data is not actually ready for read. Right?
> >
> > That code is decidedly dodgy indeed. I can only see smp_wmb() but no
> > matching barriers at all -- therefore the code is almost certainly as
> > good as not having any barriers at all.
> >
> > I would suggest you try and convert the code to smp_store_release() and
> > smp_load_acquire() while you're at it.
>
> Isn't this one of the places where we rely on control dependencies?
Then it bloody well should have a comment. But at least one side of the
fifo needs a read barrier I think. We can rely on a ctrl-dep on the
write side, where we read the head/tail values, compute space and then
conditionally allow writes to happen.
But on the read side it's all reads and ctrl-dep doesn't help anything.
On Mon, May 14, 2018 at 08:57:55AM +0200, Peter Zijlstra wrote:
> On Fri, May 11, 2018 at 09:20:53AM -0700, Paul E. McKenney wrote:
> > On Fri, May 11, 2018 at 10:32:42AM +0200, Peter Zijlstra wrote:
> > > On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote:
> > > >
> > > > Hi,
> > > >
> > > > Currently, there is no read barrier between reading the index
> > > > (kfifo.in) and fetching the real data from the fifo.
> > > >
> > > > I am afraid that will cause the vfifo is observed as not empty
> > > > however the data is not actually ready for read. Right?
> > >
> > > That code is decidedly dodgy indeed. I can only see smp_wmb() but no
> > > matching barriers at all -- therefore the code is almost certainly as
> > > good as not having any barriers at all.
> > >
> > > I would suggest you try and convert the code to smp_store_release() and
> > > smp_load_acquire() while you're at it.
> >
> > Isn't this one of the places where we rely on control dependencies?
>
> Then it bloody well should have a comment. But at least one side of the
> fifo needs a read barrier I think. We can rely on a ctrl-dep on the
> write side, where we read the head/tail values, compute space and then
> conditionally allow writes to happen.
>
> But on the read side it's all reads and ctrl-dep doesn't help anything.
Agreed, for a control depdendency to help, there does need to be a
control dependency. ;-)
Thanx, Paul