From: Steven Rostedt <rost...@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
and trigger a warning. 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 <pet...@infradead.org>
Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
---
kernel/unwind/deferred.c | 52 +++++++++++++++++++++++++++++++++-------
1 file changed, 44 insertions(+), 8 deletions(-)
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 2cbae2ada309..c5ac087d2396 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -12,6 +12,31 @@
#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.
+ */
+#if defined(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG)
+# define CAN_USE_IN_NMI 1
+static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt)
+{
+ u32 old = 0;
+
+ return try_cmpxchg(&info->id.cnt, &old, cnt);
+}
+#else
+# define CAN_USE_IN_NMI 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;
+}
+#endif
+
/* Make the cache fit in a 4K page */
#define UNWIND_MAX_ENTRIES \
((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long))
@@ -42,14 +67,13 @@ static DEFINE_PER_CPU(u32, unwind_ctx_ctr);
static u64 get_cookie(struct unwind_task_info *info)
{
u32 cnt = 1;
- u32 old = 0;
if (info->id.cpu)
return info->id.id;
/* LSB is always set to ensure 0 is an invalid value */
cnt |= __this_cpu_read(unwind_ctx_ctr) + 2;
- if (try_cmpxchg(&info->id.cnt, &old, cnt)) {
+ if (try_assign_cnt(info, cnt)) {
/* Update the per cpu counter */
__this_cpu_write(unwind_ctx_ctr, cnt);
}
@@ -167,31 +191,43 @@ static void unwind_deferred_task_work(struct
callback_head *head)
int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
{
struct unwind_task_info *info = ¤t->unwind_info;
+ long pending;
int ret;
*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.
+ * Trigger a warning to make it obvious that an architecture
+ * is using this in NMI when it should not be.
+ */
+ if (WARN_ON_ONCE(!CAN_USE_IN_NMI && in_nmi()))
+ return -EINVAL;
+
guard(irqsave)();
*cookie = get_cookie(info);
/* callback already pending? */
- if (info->pending)
+ pending = READ_ONCE(info->pending);
+ if (pending)
+ return 1;
+
+ /* Claim the work unless an NMI just now swooped in to do so. */
+ if (!try_cmpxchg(&info->pending, &pending, 1))
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;
}
--
2.47.2
