[PATCH v2 2/2] tracing/hwlat: Fix deadlock in cpuhp processing

Wei Li
2024-11-21T02:06:14Z

One "hung task" error was reported during the test, and i figured out
the deadlock scenario is as follows:

T1 [BP] | T2 [AP] | T3 [hwlatd/1] | T4
work_for_cpu_fn() | cpuhp_thread_fun() | kthread_fn() | hwlat_hotplug_workfn()
_cpu_down() | stop_cpu_kthread() | | mutex_lock(&hwlat_data.lock)
cpus_write_lock() | kthread_stop(hwlatd/1) | mutex_lock(&hwlat_data.lock) |
__cpuhp_kick_ap() | wait_for_completion() | | cpus_read_lock()

It constitutes ABBA deadlock indirectly between "cpu_hotplug_lock" and
"hwlat_data.lock", remove the mutex_lock in kthread_fn() to fix this.

Fixes: ba998f7d9531 ("trace/hwlat: Support hotplug operations")
Signed-off-by: Wei Li <liwei391@huawei.com>
---
kernel/trace/trace_hwlat.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 3bd6071441ad..7d50b5ea999a 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -108,6 +108,7 @@ static struct hwlat_data {

u64 sample_window; /* total sampling window (on+off) */
u64 sample_width; /* active sampling portion of window */
+ u64 sample_interval; /* sample_window - sample_width */

int thread_mode; /* thread mode */

@@ -370,10 +371,7 @@ static int kthread_fn(void *data)
get_sample();
local_irq_enable();

- mutex_lock(&hwlat_data.lock);
- interval = hwlat_data.sample_window - hwlat_data.sample_width;
- mutex_unlock(&hwlat_data.lock);
-
+ interval = READ_ONCE(hwlat_data.sample_interval);
do_div(interval, USEC_PER_MSEC); /* modifies interval value */

/* Always sleep for at least 1ms */
@@ -729,6 +727,13 @@ static ssize_t hwlat_mode_write(struct file *filp, const char __user *ubuf,
return ret;
}

+static void hwlat_width_window_write(u64 *val, u64 data)
+{
+ *val = data;
+ WRITE_ONCE(hwlat_data.sample_interval,
+ hwlat_data.sample_window - hwlat_data.sample_width);
+}
+
/*
* The width parameter is read/write using the generic trace_min_max_param
* method. The *val is protected by the hwlat_data lock and is upper
@@ -737,6 +742,7 @@ static ssize_t hwlat_mode_write(struct file *filp, const char __user *ubuf,
static struct trace_min_max_param hwlat_width = {
.lock = &hwlat_data.lock,
.val = &hwlat_data.sample_width,
+ .write = &hwlat_width_window_write,
.max = &hwlat_data.sample_window,
.min = NULL,
};
@@ -749,6 +755,7 @@ static struct trace_min_max_param hwlat_width = {
static struct trace_min_max_param hwlat_window = {
.lock = &hwlat_data.lock,
.val = &hwlat_data.sample_window,
+ .write = &hwlat_width_window_write,
.max = NULL,
.min = &hwlat_data.sample_width,
};
--
2.25.1