linux-kernel - Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)

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Message-ID: <20131224082931.GA20471@gmail.com>
Date:	Tue, 24 Dec 2013 09:29:31 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jason Seba <jason.seba42@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tomas Henzl <thenzl@...hat.com>, Jack Wang <xjtuwjp@...il.com>,
	Suresh Thiagarajan <Suresh.Thiagarajan@...s.com>,
	Viswas G <Viswas.G@...s.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"JBottomley@...allels.com" <JBottomley@...allels.com>,
	Vasanthalakshmi Tharmarajan 
	<Vasanthalakshmi.Tharmarajan@...s.com>,
	linux-kernel@...r.kernel.org
Subject: Re: spinlock_irqsave() && flags (Was: pm80xx: Spinlock fix)


* Oleg Nesterov <oleg@...hat.com> wrote:

> On 12/23, Ingo Molnar wrote:
> >
> > * Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > > Initially I thought that this is obviously wrong, irqsave/irqrestore
> > > assume that "flags" is owned by the caller, not by the lock. And
> > > iirc this was certainly wrong in the past.
> > >
> > > But when I look at spinlock.c it seems that this code can actually
> > > work. _irqsave() writes to FLAGS after it takes the lock, and
> > > _irqrestore() has a copy of FLAGS before it drops this lock.
> >
> > I don't think that's true: if it was then the lock would not be
> > irqsave, a hardware-irq could come in after the lock has been taken
> > and before flags are saved+disabled.
> 
> I do agree that this pattern is not safe, that is why I decided to ask.
> 
> But, unless I missed something, with the current implementation
> spin_lock_irqsave(lock, global_flags) does:
> 
> 	unsigned long local_flags;
> 
> 	local_irq_save(local_flags);
> 	spin_lock(lock);
> 
> 	global_flags = local_flags;
> 
> so the access to global_flags is actually serialized by lock.

You are right, today that's true technically because IIRC due to Sparc 
quirks we happen to return 'flags' as a return value - still it's very 
ugly and it could break anytime if we decide to do more aggressive 
optimizations and actually directly save into 'flags'.

Note that even today there's a narrow exception: on UP we happen to 
build it the other way around, so that we do:

	local_irq_save(global_flags);
	__acquire(lock);

This does not matter for any real code because on UP there is no 
physical lock and __acquire() is empty code-wise, but any compiler 
driven locking analysis tool using __attribute__ __context__(), if 
built on UP, would see the unsafe locking pattern.

Thanks,

	Ingo
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