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tracing: Wait for preempt irq delay thread to execute
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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: d16a8c3 ("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>
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Steven Rostedt (VMware) committed May 11, 2020
1 parent 9d82ccd commit 8b1fac2
Showing 1 changed file with 10 additions and 2 deletions.
12 changes: 10 additions & 2 deletions kernel/trace/preemptirq_delay_test.c
Original file line number Diff line number Diff line change
Expand Up @@ -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";
Expand All @@ -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)
Expand Down Expand Up @@ -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();
Expand All @@ -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;
}

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