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 <adobri...@gmail.com>
> ---
>
> 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) <pet...@infradead.org>
---
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], ...);
