On Mon, Jul 07, 2025 at 09:22:46PM -0400, Steven Rostedt wrote:
From: Steven Rostedt <rostedt@goodmis.org>
Make unwind_deferred_request() NMI-safe so tracers in NMI context can
call it and safely request a user space stacktrace when the task exits.
Note, this is only allowed for architectures that implement a safe
cmpxchg. If an architecture requests a deferred stack trace from NMI
context that does not support a safe NMI cmpxchg, it will get an -EINVAL.
For those architectures, they would need another method (perhaps an
irqwork), to request a deferred user space stack trace. That can be dealt
with later if one of theses architectures require this feature.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
How's this instead?
---
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -12,6 +12,40 @@
#include <linux/slab.h>
#include <linux/mm.h>
+/*
+ * For requesting a deferred user space stack trace from NMI context
+ * the architecture must support a safe cmpxchg in NMI context.
+ * For those architectures that do not have that, then it cannot ask
+ * for a deferred user space stack trace from an NMI context. If it
+ * does, then it will get -EINVAL.
+ */
+#ifdef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
+#define UNWIND_NMI_SAFE 1
+static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt)
+{
+ u32 zero = 0;
+ return try_cmpxchg(&info->id.cnt, &zero, cnt);
+}
+static inline bool test_and_set_pending(struct unwind_task_info *info)
+{
+ return info->pending || cmpxchg_local(&info->pending, 0, 1);
+}
+#else
+#define UNWIND_NMI_SAFE 0
+/* When NMIs are not allowed, this always succeeds */
+static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt)
+{
+ info->id.cnt = cnt;
+ return true;
+}
+static inline bool test_and_set_pending(struct unwind_task_info *info)
+{
+ int pending = info->pending;
+ info->pending = 1;
+ return pending;
+}
+#endif /* CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG */
+
/* Make the cache fit in a 4K page */
#define UNWIND_MAX_ENTRIES \
((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long))@@ -41,21 +75,16 @@ static DEFINE_PER_CPU(u32, unwind_ctx_ct
*/
static u64 get_cookie(struct unwind_task_info *info)
{
- u32 cpu_cnt;
- u32 cnt;
- u32 old = 0;
+ u32 cnt = 1;
if (info->id.cpu)
return info->id.id;
- cpu_cnt = __this_cpu_read(unwind_ctx_ctr);
- cpu_cnt += 2;
- cnt = cpu_cnt | 1; /* Always make non zero */
-
- if (try_cmpxchg(&info->id.cnt, &old, cnt)) {
- /* Update the per cpu counter */
- __this_cpu_write(unwind_ctx_ctr, cpu_cnt);
- }
+ /* LSB it always set to ensure 0 is an invalid value. */
+ cnt |= __this_cpu_read(unwind_ctx_ctr) + 2;
+ if (try_assign_cnt(info, cnt))
+ __this_cpu_write(unwind_ctx_ctr, cnt);
+
/* Interrupts are disabled, the CPU will always be same */
info->id.cpu = smp_processor_id() + 1; /* Must be non zero */
@@ -174,27 +203,29 @@ int unwind_deferred_request(struct unwin
*cookie = 0;
- if (WARN_ON_ONCE(in_nmi()))
- return -EINVAL;
-
if ((current->flags & (PF_KTHREAD | PF_EXITING)) ||
!user_mode(task_pt_regs(current)))
return -EINVAL;
+ /* NMI requires having safe cmpxchg operations */
+ if (WARN_ON_ONCE(!UNWIND_NMI_SAFE && in_nmi()))
+ return -EINVAL;
+
guard(irqsave)();
*cookie = get_cookie(info);
/* callback already pending? */
- if (info->pending)
+ if (test_and_set_pending(info))
return 1;
/* The work has been claimed, now schedule it. */
ret = task_work_add(current, &info->work, TWA_RESUME);
- if (WARN_ON_ONCE(ret))
+ if (WARN_ON_ONCE(ret)) {
+ WRITE_ONCE(info->pending, 0);
return ret;
+ }
- info->pending = 1;
return 0;
}