If there are more than 3 threads racing on the same address, it can
happen that 'other_info' is populated not by the thread that consumed
the calling thread's watchpoint but by one of the others.
To avoid deadlock, we have to consume 'other_info' regardless. In case
we observe that we only have information about readers, we discard the
'other_info' and skip the report.
Signed-off-by: Marco Elver <[email protected]>
---
kernel/kcsan/report.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 3bc590e6be7e3..e046dd26a2459 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -422,6 +422,26 @@ static bool prepare_report(unsigned long *flags, const volatile void *ptr,
return false;
}
+ access_type |= other_info.access_type;
+ if ((access_type & KCSAN_ACCESS_WRITE) == 0) {
+ /*
+ * This is not the other_info from the thread that
+ * consumed our watchpoint.
+ *
+ * There are concurrent races between more than 3
+ * threads on the same address. The thread that set up
+ * the watchpoint here was a read, as well as the one
+ * that is currently in other_info.
+ *
+ * It's fine if we simply omit this report, since the
+ * chances of one of the other reports including the
+ * same info is high, as well as the chances that we
+ * simply re-report the race again.
+ */
+ release_report(flags, KCSAN_REPORT_RACE_SIGNAL);
+ return false;
+ }
+
/*
* Matching & usable access in other_info: keep other_info_lock
* locked, as this thread consumes it to print the full report;
--
2.25.0.341.g760bfbb309-goog
On Mon, Feb 10, 2020 at 03:56:39PM +0100, Marco Elver wrote:
> If there are more than 3 threads racing on the same address, it can
> happen that 'other_info' is populated not by the thread that consumed
> the calling thread's watchpoint but by one of the others.
>
> To avoid deadlock, we have to consume 'other_info' regardless. In case
> we observe that we only have information about readers, we discard the
> 'other_info' and skip the report.
>
> Signed-off-by: Marco Elver <[email protected]>
Queued for testing and review, thank you!!!
Thanx, Paul
> ---
> kernel/kcsan/report.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
>
> diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
> index 3bc590e6be7e3..e046dd26a2459 100644
> --- a/kernel/kcsan/report.c
> +++ b/kernel/kcsan/report.c
> @@ -422,6 +422,26 @@ static bool prepare_report(unsigned long *flags, const volatile void *ptr,
> return false;
> }
>
> + access_type |= other_info.access_type;
> + if ((access_type & KCSAN_ACCESS_WRITE) == 0) {
> + /*
> + * This is not the other_info from the thread that
> + * consumed our watchpoint.
> + *
> + * There are concurrent races between more than 3
> + * threads on the same address. The thread that set up
> + * the watchpoint here was a read, as well as the one
> + * that is currently in other_info.
> + *
> + * It's fine if we simply omit this report, since the
> + * chances of one of the other reports including the
> + * same info is high, as well as the chances that we
> + * simply re-report the race again.
> + */
> + release_report(flags, KCSAN_REPORT_RACE_SIGNAL);
> + return false;
> + }
> +
> /*
> * Matching & usable access in other_info: keep other_info_lock
> * locked, as this thread consumes it to print the full report;
> --
> 2.25.0.341.g760bfbb309-goog
>