Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 27 Jun 2022 09:11:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Wedson Almeida Filho <wedsonaf@google.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Christian Brauner <brauner@kernel.org>,
        Tejun Heo <tj@kernel.org>, Petr Mladek <pmladek@suse.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Michal Hocko <mhocko@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: re. Spurious wakeup on a newly created kthread
Message-ID: <YrlYDppYKw6HRP1j@hirez.programming.kicks-ass.net>
References: <YraWWl+Go17uPOgR@mtj.duckdns.org>
 <CAHk-=wiC7rj1o7vTnYUPfD7YxAu09MZiZbahHqvLm9+Cgg1dFw@mail.gmail.com>
 <874k0863x8.fsf@email.froward.int.ebiederm.org>
 <CAHk-=wgTG2K3erROP19320zBN6BHVf0hRfXGdawkGR4gzrJN6w@mail.gmail.com>
 <CAHk-=whLsaRKaFKS0UffeCYYCVyP0bbiB4BTYTaXtScgu6R9yA@mail.gmail.com>
 <87pmiw1fy6.fsf@email.froward.int.ebiederm.org>
 <CAHk-=wiutNT47oNhyk_WvMj2qp4pehYFptXCUzW=u_2STLQiww@mail.gmail.com>
 <CAHk-=whX_=BNZ4kVEAu2NV3CMnhwsuYTyE65FQXUMx8VPNOAOA@mail.gmail.com>
 <87y1xkwa28.fsf@email.froward.int.ebiederm.org>
 <YrjxcR2M6AD6S9dW@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YrjxcR2M6AD6S9dW@google.com>
Precedence: bulk

On Mon, Jun 27, 2022 at 12:01:04AM +0000, Wedson Almeida Filho wrote:
> On Sat, Jun 24, 2022 at 07:19:27PM -0500, Eric W. Biederman wrote:
> > 
> > Further it is necessary for Peter Zijlstra's rewrite of the kernel
> > freezer.   As anything that isn't a special stop state (which
> > TASK_UNINTERRUPTIBLE is not) will receive a spurious wake up on when
> > thawed out.
> 
> Do you know if the current (i.e., prior to the rewrite) kernel freezer
> also sends spurious wakeups when thawing tasks?

Current freezer can thaw at random points in time, even before SMP
bringup if you're unlucky. And yes, I think it can induce 'spurious'
wakeups as well.

But really; like Linus already said upsteam, every wait loop *MUST*
already be able to deal with spurious wakeups. This is why pretty much
every wait primitive we have looks like:

	for (;;) {
		set_current_state(state);
		if (cond)
			break;
		schedule();
	}
	__set_current_state(RUNNING);

Which is immune to random wake-ups since it need @cond to make progress.
*NEVER* rely on just the wakeup itself for progress, that's buggy as
heck in lots of ways.

There are a few exceptions, but they all require special wait states and
much carefulness.