@@ -1944,6 +1944,9 @@ static void free_ret_instance(struct uprobe_task *utask,
* to-be-reused return instances for future uretprobes. If ri_timer()
* happens to be running right now, though, we fallback to safety and
* just perform RCU-delated freeing of ri.
+ * Admittedly, this is a rather simple use of seqcount, but it nicely
+ * abstracts away all the necessary memory barriers, so we use
+ * a well-supported kernel primitive here.
*/
if (raw_seqcount_try_begin(&utask->ri_seqcount, seq)) {
/* immediate reuse of ri without RCU GP is OK */
@@ -2004,12 +2007,20 @@ static void ri_timer(struct timer_list *timer)
/* RCU protects return_instance from freeing. */
guard(rcu)();
- write_seqcount_begin(&utask->ri_seqcount);
+ /*
+ * See free_ret_instance() for notes on seqcount use.
+ * We also employ raw API variants to avoid lockdep false-positive
+ * warning complaining about enabled preemption. The timer can only be
+ * invoked once for a uprobe_task. Therefore there can only be one
+ * writer. The reader does not require an even sequence count to make
+ * progress, so it is OK to remain preemptible on PREEMPT_RT.
+ */
+ raw_write_seqcount_begin(&utask->ri_seqcount);
for_each_ret_instance_rcu(ri, utask->return_instances)
hprobe_expire(&ri->hprobe, false);
- write_seqcount_end(&utask->ri_seqcount);
+ raw_write_seqcount_end(&utask->ri_seqcount);
}
static struct uprobe_task *alloc_utask(void)
Avoid a false-positive lockdep warning in PREEMPT_RT configuration when using write_seqcount_begin() in uprobe timer callback by using raw_write_* APIs. Uprobe's use of timer callback is guaranteed to not race with itself for a given uprobe_task, and as such seqcount's insistence on having preemption disabled on the writer side is irrelevant. So switch to raw_ variants of seqcount API instead of disabling preemption unnecessarily. Also, point out in the comments more explicitly why we use seqcount despite our reader side being rather simple and never retrying. We favor well-maintained kernel primitive in favor of open-coding our own memory barriers. Link: https://lore.kernel.org/bpf/CAADnVQLLOHZmPO4X_dQ+cTaSDvzdWHzA0qUqQDhLFYL3D6xPxg@mail.gmail.com/ Reported-by: Alexei Starovoitov <ast@kernel.org> Suggested-by: Sebastian Siewior <bigeasy@linutronix.de> Fixes: 8622e45b5da1 ("uprobes: Reuse return_instances between multiple uretprobes within task") Signed-off-by: Andrii Nakryiko <andrii@kernel.org> --- v1->v2: - fix comment style and s/hardirqs/preemption/ (Sebastian); - improved added comment based on Sebastian's suggestions. kernel/events/uprobes.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)