2021-09-02 02:59:13

by Jiasheng Jiang

Subject: [PATCH 10/10] seqlock: Add do_read_seqcount_retry() in front of seqcount_lockdep_reader_access() in read_seqbegin()

We have found that in the complied files seqcount_lockdep_reader_access()
appear more than 20 times, and under at least 90% circumstances
that do_read_seqcount_retry() and seqcount_lockdep_reader_access()
appear in pairs.
For example, they appear together in the irqfd_wakeup()
of the file complie from 'virt/kvm/eventfd.c'.
But we have found that in the read_seqbegin(), there is only
seqcount_lockdep_reader_access() instead of the pair.
Therefore, we consider that the do_read_seqcount_retry()
might be forgotten.

Signed-off-by: jiasheng <[email protected]>
---
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 f61e34f..14169ce 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -837,8 +837,10 @@ typedef struct {
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
-
+ unsigned int ret;
+
+ if (read_seqcount_retry(&sl->seqcount, ret))
+ ret = read_seqcount_begin(&sl->seqcount);
kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
kcsan_flat_atomic_begin();
return ret;
--
2.7.4

2021-09-02 03:01:02

by Eric Dumazet

Subject: Re: [PATCH 10/10] seqlock: Add do_read_seqcount_retry() in front of seqcount_lockdep_reader_access() in read_seqbegin()

On 9/1/21 7:16 PM, jiasheng wrote:
> We have found that in the complied files seqcount_lockdep_reader_access()
> appear more than 20 times, and under at least 90% circumstances
> that do_read_seqcount_retry() and seqcount_lockdep_reader_access()
> appear in pairs.
> For example, they appear together in the irqfd_wakeup()
> of the file complie from 'virt/kvm/eventfd.c'.
> But we have found that in the read_seqbegin(), there is only
> seqcount_lockdep_reader_access() instead of the pair.
> Therefore, we consider that the do_read_seqcount_retry()
> might be forgotten.
>
> Signed-off-by: jiasheng <[email protected]>
> ---
> 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 f61e34f..14169ce 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -837,8 +837,10 @@ typedef struct {
> */
> static inline unsigned read_seqbegin(const seqlock_t *sl)
> {
> - unsigned ret = read_seqcount_begin(&sl->seqcount);
> -
> + unsigned int ret;

ret contains garbage at this point.

Calling read_seqcount_retry() with garbage is not going to fly.

Have you really compiled and tested this patch ?

A sane compiler would have complained.

> +
> + if (read_seqcount_retry(&sl->seqcount, ret))
> + ret = read_seqcount_begin(&sl->seqcount);
> kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
> kcsan_flat_atomic_begin();
> return ret;
>