linux-kernel - Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241228152229.GC5302@redhat.com>
Date: Sat, 28 Dec 2024 16:22:30 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Manfred Spraul <manfred@...orfullife.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, viro@...iv.linux.org.uk,
	brauner@...nel.org, jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, oliver.sang@...el.com,
	ebiederm@...ssion.com, colin.king@...onical.com,
	josh@...htriplett.org, penberg@...helsinki.fi, mingo@...e.hu,
	jes@....com, hch@....de, aia21@...tab.net, arjan@...radead.org,
	jgarzik@...ox.com, neukum@...hschaft.cup.uni-muenchen.de,
	oliver@...kum.name, dada1@...mosbay.com, axboe@...nel.dk,
	axboe@...e.de, nickpiggin@...oo.com.au, dhowells@...hat.com,
	nathans@....com, rolandd@...co.com, tytso@....edu, bunk@...sta.de,
	pbadari@...ibm.com, ak@...ux.intel.com, ak@...e.de,
	davem@...emloft.net, jsipek@...sunysb.edu, jens.axboe@...cle.com,
	ramsdell@...re.org, hch@...radead.org, akpm@...ux-foundation.org,
	randy.dunlap@...cle.com, efault@....de, rdunlap@...radead.org,
	haveblue@...ibm.com, drepper@...hat.com, dm.n9107@...il.com,
	jblunck@...e.de, davidel@...ilserver.org,
	mtk.manpages@...glemail.com, linux-arch@...r.kernel.org,
	vda.linux@...glemail.com, jmorris@...ei.org, serue@...ibm.com,
	hca@...ux.ibm.com, rth@...ddle.net, lethal@...ux-sh.org,
	tony.luck@...el.com, heiko.carstens@...ibm.com, andi@...stfloor.org,
	corbet@....net, crquan@...il.com, mszeredi@...e.cz,
	miklos@...redi.hu, peterz@...radead.org, a.p.zijlstra@...llo.nl,
	earl_chew@...lent.com, npiggin@...il.com, npiggin@...e.de,
	julia@...u.dk, jaxboe@...ionio.com, nikai@...ai.net,
	dchinner@...hat.com, davej@...hat.com, npiggin@...nel.dk,
	eric.dumazet@...il.com, tim.c.chen@...ux.intel.com,
	xemul@...allels.com, tj@...nel.org, serge.hallyn@...onical.com,
	gorcunov@...nvz.org, bcrl@...ck.org, alan@...rguk.ukuu.org.uk,
	will.deacon@....com, will@...nel.org, zab@...hat.com, balbi@...com,
	gregkh@...uxfoundation.org, rusty@...tcorp.com.au,
	socketpair@...il.com, penguin-kernel@...ove.sakura.ne.jp,
	mhocko@...nel.org, axboe@...com, tglx@...utronix.de,
	mcgrof@...nel.org, linux@...inikbrodowski.net, willy@...radead.org,
	paulmck@...nel.org, kernel@...force.de,
	linux-morello@...lists.linaro.org
Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping
 processes during pipe read/write

On 12/28, Oleg Nesterov wrote:
>
> >  int __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
> >  	      int nr_exclusive, void *key)
> >  {
> > +	if (list_empty(&wq_head->head)) {
> > +		struct list_head *pn;
> > +
> > +		/*
> > +		 * pairs with spin_unlock_irqrestore(&wq_head->lock);
> > +		 * We actually do not need to acquire wq_head->lock, we just
> > +		 * need to be sure that there is no prepare_to_wait() that
> > +		 * completed on any CPU before __wake_up was called.
> > +		 * Thus instead of load_acquiring the spinlock and dropping
> > +		 * it again, we load_acquire the next list entry and check
> > +		 * that the list is not empty.
> > +		 */
> > +		pn = smp_load_acquire(&wq_head->head.next);
> > +
> > +		if(pn == &wq_head->head)
> > +			return 0;
> > +	}
>
> Too subtle for me ;)
>
> I have some concerns, but I need to think a bit more to (try to) actually
> understand this change.

If nothing else, consider

	int CONDITION;
	wait_queue_head_t WQ;

	void wake(void)
	{
		CONDITION = 1;
		wake_up(WQ);
	}

	void wait(void)
	{
		DEFINE_WAIT_FUNC(entry, woken_wake_function);

		add_wait_queue(WQ, entry);
		if (!CONDITION)
			wait_woken(entry, ...);
		remove_wait_queue(WQ, entry);
	}

this code is correct even if LOAD(CONDITION) can leak into the critical
section in add_wait_queue(), so CPU running wait() can actually do

		// add_wait_queue
		spin_lock(WQ->lock);
		LOAD(CONDITION);	// false!
		list_add(entry, head);
		spin_unlock(WQ->lock);

		if (!false)		// result of the LOAD above
			wait_woken(entry, ...);

Now suppose that another CPU executes wake() between LOAD(CONDITION)
and list_add(entry, head). With your patch wait() will miss the event.
The same for __pollwait(), I think...

No?

Oleg.