linux-kernel - Re: [PATCH] md: don't use flush_signals in userspace processes

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Date:   Fri, 09 Jun 2017 11:55:03 +1000
From:   NeilBrown <neilb@...e.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Shaohua Li <shli@...nel.org>, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] md: don't use flush_signals in userspace processes

On Thu, Jun 08 2017, Mikulas Patocka wrote:

> On Thu, 8 Jun 2017, NeilBrown wrote:
>
>> On Wed, Jun 07 2017, Mikulas Patocka wrote:
>> 
>> > The function flush_signals clears all pending signals for the process. It
>> > may be used by kernel threads when we need to prepare a kernel thread for
>> > responding to signals. However using this function for an userspaces
>> > processes is incorrect - clearing signals without the program expecting it
>> > can cause misbehavior.
>> >
>> > The raid1 and raid5 code uses flush_signals in its request routine because
>> > it wants to prepare for an interruptible wait. This patch drops
>> > flush_signals and uses sigprocmask instead to block all signals (including
>> > SIGKILL) around the schedule() call. The signals are not lost, but the
>> > schedule() call won't respond to them.
>> >
>> > Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
>> > Cc: stable@...r.kernel.org
>> 
>> Thanks for catching that!
>> 
>> Acked-by: NeilBrown <neilb@...e.com>
>> 
>> NeilBrown
>
> BTW. why does md_thread do "allow_signal(SIGKILL)" and then
> "if (signal_pending(current)) flush_signals(current)"?
>
> Does userspace really send SIGKILL to MD kernel threads? The SIGKILL will 
> be lost when flush_signals is called, so it looks quite dubious.
>

This is for md_check_recovery() which does do something on a signal.
Chances are good that it will get to handle the signal before
md_thread() flushed them, but not guaranteed.  I could be improved I
guess.

Or maybe it could be discarded - the md_check_recovery() thing.
The idea was that if you alt-sysrq-K to kill all processes, md arrays
would go into immediate-safe-mode where the metadata is marked clean
immediately after writes finish, rather than waiting a few seconds.  The
chance of having a clean array after shutdown is hopefully improved.

I've never actually used this though, and I doubt many people know about
it.  And bitmaps make it fairly pointless.
So I wouldn't object much if
	allow_signal(SIGKILL);
and
	if (signal_pending(current)) {
		if (mddev->pers->sync_request && !mddev->external) {
			pr_debug("md: %s in immediate safe mode\n",
				 mdname(mddev));
			mddev->safemode = 2;
		}
		flush_signals(current);
	}

were removed.

Thanks,
NeilBrown

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