[PATCH] tracing: Fix context switch counter truncation

[PATCH] tracing: Fix context switch counter truncation

[Usama Arif]

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.

Fixes: 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read user space")
Cc: stable@vger.kernel.org
Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 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 1146b83b711a..412a8daf2162 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6188,7 +6188,7 @@ 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;
 
-- 
2.53.0-Meta

Re: [PATCH] tracing: Fix context switch counter truncation

[Masami Hiramatsu]

On Fri, 17 Jul 2026 10:32:52 -0700
Usama Arif <usama.arif@linux.dev> wrote:

> 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.

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks!

> 
> Fixes: 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read user space")
> Cc: stable@vger.kernel.org
> Reported-by: Breno Leitao <leitao@debian.org>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  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 1146b83b711a..412a8daf2162 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6188,7 +6188,7 @@ 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;
>  
> -- 
> 2.53.0-Meta
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>