Hi,
I am looking at memory ordering and a question hit me.
I was looking at the kfifo code. kfifo_put() has a barrier:
)[__kfifo->in & __tmp->kfifo.mask] = \
(typeof(*__tmp->type))__val; \
smp_wmb(); \
__kfifo->in++; \
Looking at kfifo_get()
__ret = !kfifo_is_empty(__tmp); \
if (__ret) { \
*(typeof(__tmp->type))__val = \
(__is_kfifo_ptr(__tmp) ? \
A thought struck me. There is no corresponding barrier. I cannot
help myself, but I think there needs to be a smp_read_barrier_depends()
between reading kfifo->in (in kfifo_is empty) and reading val.
What do you think?
Regards
Oliver
On Wed, 16 Apr 2014, Oliver Neukum wrote:
> Hi,
>
> I am looking at memory ordering and a question hit me.
> I was looking at the kfifo code. kfifo_put() has a barrier:
>
> )[__kfifo->in & __tmp->kfifo.mask] = \
> (typeof(*__tmp->type))__val; \
> smp_wmb(); \
> __kfifo->in++; \
>
> Looking at kfifo_get()
>
> __ret = !kfifo_is_empty(__tmp); \
> if (__ret) { \
> *(typeof(__tmp->type))__val = \
> (__is_kfifo_ptr(__tmp) ? \
>
> A thought struck me. There is no corresponding barrier. I cannot
> help myself, but I think there needs to be a smp_read_barrier_depends()
> between reading kfifo->in (in kfifo_is empty) and reading val.
> What do you think?
I think you are right.
In addition, the following code in kfifo_get() does this:
*(typeof(__tmp->type))__val = \
(__is_kfifo_ptr(__tmp) ? \
((typeof(__tmp->type))__kfifo->data) : \
(__tmp->buf) \
)[__kfifo->out & __tmp->kfifo.mask]; \
smp_wmb(); \
__kfifo->out++; \
It looks like the smp_wmb() should really be smp_mb(), because it
separates the _read_ for val from the _write_ of kfifo->out.
Alan Stern
On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> In addition, the following code in kfifo_get() does this:
>
> *(typeof(__tmp->type))__val = \
> (__is_kfifo_ptr(__tmp) ? \
> ((typeof(__tmp->type))__kfifo->data) : \
> (__tmp->buf) \
> )[__kfifo->out & __tmp->kfifo.mask]; \
> smp_wmb(); \
> __kfifo->out++; \
>
> It looks like the smp_wmb() should really be smp_mb(), because it
> separates the _read_ for val from the _write_ of kfifo->out.
But where is kfifo->out read at all?
Regards
Oliver
On Thu, 17 Apr 2014, Oliver Neukum wrote:
> On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
>
> > In addition, the following code in kfifo_get() does this:
> >
> > *(typeof(__tmp->type))__val = \
> > (__is_kfifo_ptr(__tmp) ? \
> > ((typeof(__tmp->type))__kfifo->data) : \
> > (__tmp->buf) \
> > )[__kfifo->out & __tmp->kfifo.mask]; \
> > smp_wmb(); \
> > __kfifo->out++; \
> >
> > It looks like the smp_wmb() should really be smp_mb(), because it
> > separates the _read_ for val from the _write_ of kfifo->out.
>
> But where is kfifo->out read at all?
It is read in kfifo_put(), inside the call to kfifo_is_full() ->
kfifo_len().
Alan Stern
On Thu, 2014-04-17 at 11:50 -0400, Alan Stern wrote:
> On Thu, 17 Apr 2014, Oliver Neukum wrote:
>
> > On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> >
> > > In addition, the following code in kfifo_get() does this:
> > >
> > > *(typeof(__tmp->type))__val = \
> > > (__is_kfifo_ptr(__tmp) ? \
> > > ((typeof(__tmp->type))__kfifo->data) : \
> > > (__tmp->buf) \
> > > )[__kfifo->out & __tmp->kfifo.mask]; \
> > > smp_wmb(); \
> > > __kfifo->out++; \
> > >
> > > It looks like the smp_wmb() should really be smp_mb(), because it
> > > separates the _read_ for val from the _write_ of kfifo->out.
> >
> > But where is kfifo->out read at all?
>
> It is read in kfifo_put(), inside the call to kfifo_is_full() ->
> kfifo_len().
I had overlooked that. In that case kfifo_put() also needs
smp_read_barrier_depends()
Regards
Oliver
On Thu, 2014-04-17 at 11:50 -0400, Alan Stern wrote:
> On Thu, 17 Apr 2014, Oliver Neukum wrote:
>
> > On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> >
> > > In addition, the following code in kfifo_get() does this:
> > >
> > > *(typeof(__tmp->type))__val = \
> > > (__is_kfifo_ptr(__tmp) ? \
> > > ((typeof(__tmp->type))__kfifo->data) : \
> > > (__tmp->buf) \
> > > )[__kfifo->out & __tmp->kfifo.mask]; \
> > > smp_wmb(); \
> > > __kfifo->out++; \
> > >
> > > It looks like the smp_wmb() should really be smp_mb(), because it
> > > separates the _read_ for val from the _write_ of kfifo->out.
> >
> > But where is kfifo->out read at all?
>
> It is read in kfifo_put(), inside the call to kfifo_is_full() ->
> kfifo_len().
I had overlooked that. In that case kfifo_put() also needs
smp_read_barrier_depends()
Regards
Oliver
On Fri, 18 Apr 2014, Oliver Neukum wrote:
> On Thu, 2014-04-17 at 11:50 -0400, Alan Stern wrote:
> > On Thu, 17 Apr 2014, Oliver Neukum wrote:
> >
> > > On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> > >
> > > > In addition, the following code in kfifo_get() does this:
> > > >
> > > > *(typeof(__tmp->type))__val = \
> > > > (__is_kfifo_ptr(__tmp) ? \
> > > > ((typeof(__tmp->type))__kfifo->data) : \
> > > > (__tmp->buf) \
> > > > )[__kfifo->out & __tmp->kfifo.mask]; \
> > > > smp_wmb(); \
> > > > __kfifo->out++; \
> > > >
> > > > It looks like the smp_wmb() should really be smp_mb(), because it
> > > > separates the _read_ for val from the _write_ of kfifo->out.
> > >
> > > But where is kfifo->out read at all?
> >
> > It is read in kfifo_put(), inside the call to kfifo_is_full() ->
> > kfifo_len().
>
> I had overlooked that. In that case kfifo_put() also needs
> smp_read_barrier_depends()
I don't think so.
kfifo_put() uses the read of kfifo->out to determine whether to perform
the store; i.e., it won't store the data if the kfifo is already full.
No architectures do speculative writes, so the data can never get
stored before kfifo->out is read, even without any memory barriers.
smp_read_barrier_depends() is meant to prevent speculative _reads_, or
any similar mechanism.
Alan Stern
On Wed, 2014-04-16 at 11:26 -0400, Alan Stern wrote:
> On Wed, 16 Apr 2014, Oliver Neukum wrote:
>
> > Hi,
> >
> > I am looking at memory ordering and a question hit me.
> > I was looking at the kfifo code. kfifo_put() has a barrier:
> >
> > )[__kfifo->in & __tmp->kfifo.mask] = \
> > (typeof(*__tmp->type))__val; \
> > smp_wmb(); \
> > __kfifo->in++; \
> >
> > Looking at kfifo_get()
> >
> > __ret = !kfifo_is_empty(__tmp); \
> > if (__ret) { \
> > *(typeof(__tmp->type))__val = \
> > (__is_kfifo_ptr(__tmp) ? \
> >
> > A thought struck me. There is no corresponding barrier. I cannot
> > help myself, but I think there needs to be a smp_read_barrier_depends()
> > between reading kfifo->in (in kfifo_is empty) and reading val.
> > What do you think?
>
> I think you are right.
>
> In addition, the following code in kfifo_get() does this:
>
> *(typeof(__tmp->type))__val = \
> (__is_kfifo_ptr(__tmp) ? \
> ((typeof(__tmp->type))__kfifo->data) : \
> (__tmp->buf) \
> )[__kfifo->out & __tmp->kfifo.mask]; \
> smp_wmb(); \
> __kfifo->out++; \
>
> It looks like the smp_wmb() should really be smp_mb(), because it
> separates the _read_ for val from the _write_ of kfifo->out.
On the third hand, I now think wmb() is sufficient, because
there's also a write to __val. It does depend on the read
of buf[out & mask], but if no CPU does speculative writes
it must be correct.
Regards
Oliver
On Mon, 21 Apr 2014, Oliver Neukum wrote:
> > In addition, the following code in kfifo_get() does this:
> >
> > *(typeof(__tmp->type))__val = \
> > (__is_kfifo_ptr(__tmp) ? \
> > ((typeof(__tmp->type))__kfifo->data) : \
> > (__tmp->buf) \
> > )[__kfifo->out & __tmp->kfifo.mask]; \
> > smp_wmb(); \
> > __kfifo->out++; \
> >
> > It looks like the smp_wmb() should really be smp_mb(), because it
> > separates the _read_ for val from the _write_ of kfifo->out.
>
> On the third hand, I now think wmb() is sufficient, because
> there's also a write to __val. It does depend on the read
> of buf[out & mask], but if no CPU does speculative writes
> it must be correct.
You are right; I missed that. Good point.
Alan Stern