Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate

From: Ming Lei <ming.lei@redhat.com>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	tj@kernel.org, gregkh@linuxfoundation.org,
	akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org,
	shuah@kernel.org, bvanassche@acm.org, dan.j.williams@intel.com,
	joe@perches.com, tglx@linutronix.de, keescook@chromium.org,
	rostedt@goodmis.org, linux-spdx@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate
Date: Fri, 22 Oct 2021 08:05:18 +0800	[thread overview]
Message-ID: <YXIAPtvxW98lyM9r@T590> (raw)
In-Reply-To: <YXGg95OcaR+5ktAZ@bombadil.infradead.org>

On Thu, Oct 21, 2021 at 10:18:47AM -0700, Luis Chamberlain wrote:
> On Thu, Oct 21, 2021 at 08:39:05AM +0800, Ming Lei wrote:
> > On Wed, Oct 20, 2021 at 08:48:04AM -0700, Luis Chamberlain wrote:
> > > A second series of tests is hitting CTRL-C on either randonly and
> > > restarting testing once again randomly.
> > 
> > ltp/zram02.sh has cleanup handler via trap to clean everything(swapoff/umount/reset/
> > rmmod), ctrl-c will terminate current forground task and cause shell to run the
> > cleanup handler first, but further 'ctrl-c' will terminate the cleanup handler,
> > then the cleanup won't be done completely, such as zram disk is left as swap
> > device and zram can't be unloaded. The idea can be observed via the following
> > script:
> > 
> > 	#!/bin/bash
> > 	trap 'echo "enter trap"; sleep 20; echo "exit trap";' INT
> > 	sleep 30
> > 
> > After the above script is run foreground, when 1st ctrl-c is pressed, 'sleep 30'
> > is terminated, then the trap command is run, so you can see "enter trap"
> > dumped. Then if you pressed 2nd ctrl-c, 'sleep 20' is terminated immediately.
> > So 'swapoff' from zram02.sh's trap function can be terminated in this way.
> > 
> > zram disk being left as swap disk can be observed with your patch too
> > after terminating via multiple ctrl-c which has to be done this way because
> > the test is dead loop.
> > 
> > So it is hard to cleanup everything completely after multiple 'CTRL-C' is
> > involved, and it should be impossible. It needs violent multiple ctrl-c to
> > terminate the dealoop test.
> > 
> > So it isn't reasonable to expect that zram can be always unloaded successfully
> > after the test script is terminated via multiple ctrl-c.
> 
> For the life of me, I do not run into these issue with my patch. But
> with yours I had.
> 
> To be clear, I run zram02.sh on two terminals. Then to interrupt I just leave
> CTRL-C pressed to issue multiple terminations until the script is done
> on each terminal at a time, until I see both have completed.
> 
> I repeat the same test, noting always that when I start one one terminal
> the test is succeeding. And also when I cancel completely one script the
> test continue fine without issue.

As I explained wrt. shell's trap, this issue won't be avoided from
userspace because trap function can be terminated by ctrl-c too,
otherwise one shell script may not be terminated at all.

The unclean shutdown can be observed in single 'while true; do zram02.sh; done'
too on both your patches and mine.

Also it is insane to write write test in a deadloop, and people seldom
do that, not see such way in either blktests/xfstests.

I you limit completion time of this test in long enough time(one or
several hours) or big enough loops, I believe it can be done cleanly,
such as:

cnt=0
MAX=10000
while [ $cnt -lt $MAX ]; do
	PATH=$PATH:$PWD:$PWD/../../../lib/ ./zram02.sh;
done

Thanks,
Ming