Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread

From: Oleg Nesterov <oleg@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <jstultz@google.com>, Marco Elver <elver@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	Dmitry Vyukov <dvyukov@google.com>,
	kasan-dev@googlegroups.com, Edward Liaw <edliaw@google.com>,
	Carlos Llamas <cmllamas@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread
Date: Thu, 4 Apr 2024 16:54:08 +0200	[thread overview]
Message-ID: <20240404145408.GD7153@redhat.com> (raw)
In-Reply-To: <87frw2axv0.ffs@tglx>

On 04/04, Thomas Gleixner wrote:
>
> IOW, we cannot test this reliably at all with the current approach.

Agreed!

So how about a REALLY SIMPLE test-case below?

Lacks error checking, should be updated to match tools/testing/selftests.

Without commit bcb7ee79029dca assert(sig_cnt > SIG_CNT) fails, the very
1st tick wakes the leader up.

With that commit it doesn't fail.

Oleg.

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <pthread.h>
#include <time.h>
#include <assert.h>

#define SIG_CNT	100
static volatile int sig_cnt;

static void alarm_func(int sig)
{
	++sig_cnt;
}

static void *thread_func(void *arg)
{
	// one second before the 1st tick to ensure the leader sleeps
	struct itimerspec val = {
		.it_value.tv_sec = 1,
		.it_value.tv_nsec = 0,
		.it_interval.tv_sec = 0,
		.it_interval.tv_nsec = 1000 * 1000,
	};
	timer_t id;

	timer_create(CLOCK_PROCESS_CPUTIME_ID, NULL, &id);
	timer_settime(id, 0, &val, NULL);

	while (sig_cnt < SIG_CNT)
		;

	// wake up the leader
	kill(getpid(), SIGALRM);

	return NULL;
}

int main(void)
{
	pthread_t thread;

	signal(SIGALRM, alarm_func);

	pthread_create(&thread, NULL, thread_func, NULL);

	pause();

	assert(sig_cnt > SIG_CNT); // likely SIG_CNT + 1

	return 0;
}