From 8f76afb9b114bee1c1251e0e52553e9dc7c59f20 Mon Sep 17 00:00:00 2001 From: Usama Arif Date: Fri, 17 Jul 2026 10:32:52 -0700 Subject: [PATCH] tracing: Fix context switch counter truncation trace_user_fault_read() samples nr_context_switches_cpu() before enabling preemption and retries the user copy if the counter changes. The helper returns unsigned long long because rq->nr_switches is u64, but the saved value is unsigned int. Once a CPU has performed 2^32 context switches, assigning the counter to cnt discards its upper bits. The comparison after the copy promotes cnt back to unsigned long long, but the lost bits remain zero, so it reports a change even when the task was never scheduled out. Every retry then fails the same way until the 100-try guard warns and the user copy is abandoned. This affects long-running systems and workloads with high context-switch rates. A CPU switching 1,000 times per second takes about 50 days. Store the sampled count in unsigned long long so the full value is preserved. Cc: stable@vger.kernel.org Fixes: 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read user space") Link: https://patch.msgid.link/20260717173252.3431565-1-usama.arif@linux.dev Reported-by: Breno Leitao Signed-off-by: Usama Arif Acked-by: Masami Hiramatsu (Google) Reviewed-by: Breno Leitao Signed-off-by: Steven Rostedt --- kernel/trace/trace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index 18710c190c92..01a5e87af299 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -6187,7 +6187,7 @@ char *trace_user_fault_read(struct trace_user_buf_info *tinfo, { int cpu = smp_processor_id(); char *buffer = per_cpu_ptr(tinfo->tbuf, cpu)->buf; - unsigned int cnt; + unsigned long long cnt; int trys = 0; int ret;