Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

2016-05-27 Thread Peter Zijlstra 
On Sat, May 21, 2016 at 11:14:49PM +0300, Alexey Dobriyan wrote:
> lockless_dereference() is supposed to take pointer not integer.
> 
> Signed-off-by: Alexey Dobriyan 
> ---
> 
>  include/linux/seqlock.h |4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -277,7 +277,7 @@ static inline void raw_write_seqcount_barrier(seqcount_t 
> *s)
>  
>  static inline int raw_read_seqcount_latch(seqcount_t *s)
>  {
> - return lockless_dereference(s->sequence);
> + return lockless_dereference(s)->sequence;
>  }
>  
>  /**
> @@ -331,7 +331,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
>   *   unsigned seq, idx;
>   *
>   *   do {
> - *   seq = lockless_dereference(latch->seq);
> + *   seq = lockless_dereference(latch)->seq;
>   *
>   *   idx = seq & 0x01;
>   *   entry = data_query(latch->data[idx], ...);

So while the code was dubious; I it is now wrong, but my head hurts.

I'll queue the below, TJs per-cpu change and the lockless_dereference()
void * cast trick.


---
Subject: seqcount: Re-fix raw_read_seqcount_latch()

Commit 50755bc1c305 ("seqlock: fix raw_read_seqcount_latch()") broke
raw_read_seqcount_latch().

If you look at the comment that was modified; the thing that changes is
the seq count, not the latch pointer.

 * void latch_modify(struct latch_struct *latch, ...)
 * {
 *  smp_wmb();  <- Ensure that the last data[1] update is visible
 *  latch->seq++;
 *  smp_wmb();  <- Ensure that the seqcount update is visible
 *
 *  modify(latch->data[0], ...);
 *
 *  smp_wmb();  <- Ensure that the data[0] update is visible
 *  latch->seq++;
 *  smp_wmb();  <- Ensure that the seqcount update is visible
 *
 *  modify(latch->data[1], ...);
 * }
 *
 * The query will have a form like:
 *
 * struct entry *latch_query(struct latch_struct *latch, ...)
 * {
 *  struct entry *entry;
 *  unsigned seq, idx;
 *
 *  do {
 *  seq = lockless_dereference(latch->seq);

So here we have:

seq = READ_ONCE(latch->seq);
smp_read_barrier_depends();

Which is exactly what we want; the new code:

seq = ({ p = READ_ONCE(latch);
 smp_read_barrier_depends(); p })->seq;

is just wrong; because it looses the volatile read on seq, which can now
be torn or worse 'optimized'. And the read_depend barrier is also placed
wrong, we want it after the load of seq, to match the above data[]
up-to-date wmb()s.

Such that when we dereference latch->data[] below, we're guaranteed to
observe the right data.

 *
 *  idx = seq & 0x01;
 *  entry = data_query(latch->data[idx], ...);
 *
 *  smp_rmb();
 *  } while (seq != latch->seq);
 *
 *  return entry;
 * }

So yes, not passing a pointer is not pretty, but the code was correct,
and isn't anymore now.

Change to explicit READ_ONCE()+smp_read_barrier_depends() to avoid
confusion and allow strict lockless_dereference() checking.

Fixes: 50755bc1c305 ("seqlock: fix raw_read_seqcount_latch()")
Signed-off-by: Peter Zijlstra (Intel) 
---
 include/linux/seqlock.h | 6 --
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 7973a821ac58..f3db247cebc8 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -277,7 +277,9 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
 
 static inline int raw_read_seqcount_latch(seqcount_t *s)
 {
-   return lockless_dereference(s)->sequence;
+   int seq = READ_ONCE(s->sequence);
+   smp_read_barrier_depends();
+   return seq;
 }
 
 /**
@@ -331,7 +333,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
  * unsigned seq, idx;
  *
  * do {
- * seq = lockless_dereference(latch)->seq;
+ * seq = raw_read_seqcount_latch(&latch->seq);
  *
  * idx = seq & 0x01;
  * entry = data_query(latch->data[idx], ...);

Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

2016-05-25 Thread Tejun Heo 
Hello,

On Mon, May 23, 2016 at 11:36:18AM +0200, Peter Zijlstra wrote:
> > include/linux/percpu-refcount.h:146:36: warning: initialization makes 
> > pointer from integer without a cast [-Wint-conversion]
> >   percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
> 
> TJ; would you prefer casting or not using lockless_dereference() here?

Casting is nasty - *(unsigned long __percpu **)& - because the macro
expects an lvalue.  I think it'd be better to revert to opencoding
READ_ONCE() and barrier there.  It's a pretty special case anyway.

Thanks.

-- 
tejun

Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

2016-05-23 Thread Peter Zijlstra 
On Sun, May 22, 2016 at 09:50:40PM +0300, Alexey Dobriyan wrote:
> On Sun, May 22, 2016 at 12:48:27PM +0200, Peter Zijlstra wrote:
> > On Sat, May 21, 2016 at 11:14:49PM +0300, Alexey Dobriyan wrote:
> > > lockless_dereference() is supposed to take pointer not integer.
> > 
> > Urgh :/
> > 
> > Is there any way we can make lockless_dereference() issue a warning if
> > we don't feed it a pointer?
> > 
> > Would something like so work? All pointer types should silently cast to
> > void * while integer (and others) should refuse to.
> 
> This works (and spammy enough in case of seqlock, which is good)
> but not for "unsigned long":
> 
>   include/linux/percpu-refcount.h:146:36: warning: initialization makes 
> pointer from integer without a cast [-Wint-conversion]
> percpu_ptr = lockless_dereference(ref->percpu_count_ptr);

TJ; would you prefer casting or not using lockless_dereference() here?

> 
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -544,6 +544,7 @@ static __always_inline void __write_once_size(volatile 
> > void *p, void *res, int s
> >   */
> >  #define lockless_dereference(p) \
> >  ({ \
> > +   __maybe_unused void * _p2 = p; \
> > typeof(p) _p1 = READ_ONCE(p); \
> > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
> > (_p1); \

Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

2016-05-22 Thread Alexey Dobriyan 
On Sun, May 22, 2016 at 12:48:27PM +0200, Peter Zijlstra wrote:
> On Sat, May 21, 2016 at 11:14:49PM +0300, Alexey Dobriyan wrote:
> > lockless_dereference() is supposed to take pointer not integer.
> 
> Urgh :/
> 
> Is there any way we can make lockless_dereference() issue a warning if
> we don't feed it a pointer?
> 
> Would something like so work? All pointer types should silently cast to
> void * while integer (and others) should refuse to.

This works (and spammy enough in case of seqlock, which is good)
but not for "unsigned long":

include/linux/percpu-refcount.h:146:36: warning: initialization makes 
pointer from integer without a cast [-Wint-conversion]
  percpu_ptr = lockless_dereference(ref->percpu_count_ptr);

> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -544,6 +544,7 @@ static __always_inline void __write_once_size(volatile 
> void *p, void *res, int s
>   */
>  #define lockless_dereference(p) \
>  ({ \
> + __maybe_unused void * _p2 = p; \
>   typeof(p) _p1 = READ_ONCE(p); \
>   smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
>   (_p1); \

Re: [PATCH] seqlock: fix raw_read_seqcount_latch()

2016-05-22 Thread Peter Zijlstra 
On Sat, May 21, 2016 at 11:14:49PM +0300, Alexey Dobriyan wrote:
> lockless_dereference() is supposed to take pointer not integer.

Urgh :/

Is there any way we can make lockless_dereference() issue a warning if
we don't feed it a pointer?

Would something like so work? All pointer types should silently cast to
void * while integer (and others) should refuse to.

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index b5ff9881bef8..8886de704d33 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -544,6 +544,7 @@ static __always_inline void __write_once_size(volatile void 
*p, void *res, int s
  */
 #define lockless_dereference(p) \
 ({ \
+   __maybe_unused void * _p2 = p; \
typeof(p) _p1 = READ_ONCE(p); \
smp_read_barrier_depends(); /* Dependency order vs. p above. */ \
(_p1); \

[PATCH] seqlock: fix raw_read_seqcount_latch()

2016-05-21 Thread Alexey Dobriyan 
lockless_dereference() is supposed to take pointer not integer.

Signed-off-by: Alexey Dobriyan 
---

 include/linux/seqlock.h |4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -277,7 +277,7 @@ static inline void raw_write_seqcount_barrier(seqcount_t *s)
 
 static inline int raw_read_seqcount_latch(seqcount_t *s)
 {
-   return lockless_dereference(s->sequence);
+   return lockless_dereference(s)->sequence;
 }
 
 /**
@@ -331,7 +331,7 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
  * unsigned seq, idx;
  *
  * do {
- * seq = lockless_dereference(latch->seq);
+ * seq = lockless_dereference(latch)->seq;
  *
  * idx = seq & 0x01;
  * entry = data_query(latch->data[idx], ...);