linux-kernel - Re: INFO: rcu detected stall in io_playback

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Date:   Sun, 8 Apr 2018 22:26:21 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     dvyukov@...gle.com
Cc:     tiwai@...e.com, viro@...iv.linux.org.uk,
        syzbot+4f2016cf5185da7759dc@...kaller.appspotmail.com,
        alsa-devel@...a-project.org, dan.carpenter@...cle.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        o-takashi@...amocchi.jp, perex@...ex.cz,
        syzkaller-bugs@...glegroups.com
Subject: Re: INFO: rcu detected stall in io_playback_transfer

Dmitry Vyukov wrote:
> Takashi has already fixed this. See this thread:
> https://groups.google.com/forum/#!searchin/syzkaller-bugs/%22INFO$3A$20rcu$20detected$20stall$20in$20snd_pcm_oss_write3$20(2)%22%7Csort:date/syzkaller-bugs/D2xWV7WTRDk/sCw_gImmCAAJ

Yes, I noticed it just before I post this. But the reason I posted this anyway
is that I feel that there is some other bug revealed by this reproducer.
How can Takashi' patch explain my observation (with linux-next-20180406) ?

/* Hits the stall */
int main(int argc, char *argv[])
{
	const int fd = open("/dev/dsp1", O_RDWR);
	int frag = (0 << 16) | 0;
	char buf[48] = { };
	unshare(CLONE_NEWNS);
	ioctl(fd, SNDCTL_DSP_SETFRAGMENT, &frag);
	write(fd, buf, sizeof(buf));
	return 0;
}

/* Does not hit the stall */
int main(int argc, char *argv[])
{
	const int fd = open("/dev/dsp1", O_RDWR);
	int frag = (0 << 16) | 0;
	char buf[48] = { };
	ioctl(fd, SNDCTL_DSP_SETFRAGMENT, &frag);
	write(fd, buf, sizeof(buf));
	return 0;
}

/* Does not hit the stall */
int main(int argc, char *argv[])
{
	const int fd = open("/dev/dsp1", O_RDWR);
	int frag = (0 << 16) | 0;
	char buf[48] = { };
	unshare(CLONE_NEWNS);
	ioctl(fd, SNDCTL_DSP_SETFRAGMENT, &frag);
	write(fd, buf, sizeof(buf));
	close(fd);
	return 0;
}