From: "Steven Rostedt (VMware)" rostedt@goodmis.org

A bug report was posted that running the preempt irq delay module on a slow machine, and removing it quickly could lead to the thread created by the modlue to execute after the module is removed, and this could cause the kernel to crash. The fix for this was to call kthread_stop() after creating the thread to make sure it finishes before allowing the module to be removed.

Now this caused the opposite problem on fast machines. What now happens is the kthread_stop() can cause the kthread never to execute and the test never to run. To fix this, add a completion and wait for the kthread to execute, then wait for it to end.

This issue caused the ftracetest selftests to fail on the preemptirq tests.

Link: https://lore.kernel.org/r/20200510114210.15d9e4af@oasis.local.home

Cc: stable@vger.kernel.org Fixes: d16a8c31077e ("tracing: Wait for preempt irq delay thread to finish") Reviewed-by: Joel Fernandes (Google) joel@joelfernandes.org Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/preemptirq_delay_test.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/preemptirq_delay_test.c b/kernel/trace/preemptirq_delay_test.c index c4c86de63cf9..312d1a0ca3b6 100644 --- a/kernel/trace/preemptirq_delay_test.c +++ b/kernel/trace/preemptirq_delay_test.c @@ -16,6 +16,7 @@ #include <linux/printk.h> #include <linux/string.h> #include <linux/sysfs.h> +#include <linux/completion.h>

static ulong delay = 100; static char test_mode[12] = "irq"; @@ -28,6 +29,8 @@ MODULE_PARM_DESC(delay, "Period in microseconds (100 us default)"); MODULE_PARM_DESC(test_mode, "Mode of the test such as preempt, irq, or alternate (default irq)"); MODULE_PARM_DESC(burst_size, "The size of a burst (default 1)");

+static struct completion done; + #define MIN(x, y) ((x) < (y) ? (x) : (y))

static void busy_wait(ulong time) @@ -114,6 +117,8 @@ static int preemptirq_delay_run(void *data) for (i = 0; i < s; i++) (testfuncs[i])(i);

+ complete(&done); + set_current_state(TASK_INTERRUPTIBLE); while (!kthread_should_stop()) { schedule(); @@ -128,15 +133,18 @@ static int preemptirq_delay_run(void *data) static int preemptirq_run_test(void) { struct task_struct *task; - char task_name[50];

+ init_completion(&done); + snprintf(task_name, sizeof(task_name), "%s_test", test_mode); task = kthread_run(preemptirq_delay_run, NULL, task_name); if (IS_ERR(task)) return PTR_ERR(task); - if (task) + if (task) { + wait_for_completion(&done); kthread_stop(task); + } return 0; }