linux-kernel - Re: [PATCH v2 21/29] nios2: Futex operations

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Date:	Fri, 18 Jul 2014 11:42:34 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arnd Bergmann <arnd@...db.de>
cc:	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH v2 21/29] nios2: Futex operations

On Fri, 18 Jul 2014, Arnd Bergmann wrote:
> On Friday 18 July 2014 14:07:42 Ley Foon Tan wrote:
> > On Thu, Jul 17, 2014 at 7:07 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > > The get_user/put_user functions really need to be annotated might_fault(),
> > > because that's what they do.
> > >
> > > The whole point of get_user() is to access an unchecked user space
> > > pointer, which  can do a number of things based on what the pointer
> > > points to:
> > >
> > > - access a user space variable that resides in memory
> > > - access a kernel pointer and fail because of the access_ok()
> > >   check
> > > - access a user space pointer that is not mapped and return
> > >   through the __ex_table fixup.
> > > - access a user space pointer that has a valid VMA but not PTE,
> > >   causing a page fault to be resolved.
> > >
> > > It's the last case that breaks here.
> > So, do you mean that we can't use get_user/put_user in futex support?
> > BTW, some architectures like sh,parisc, m68k use get_user in futex
> > function as well.
> > Any recommendation way to support futex if we can't use get_user.
> > Note, nios2 doesn't have atomic instruction.
> > Thanks.
> 
> I looked at it again now and I'm no longer sure about my initial
> interpretation. The way it seems to work is that pagefault_disable()
> turns the case I mentioned into a simple error through the fixup,
> so we return -EFAULT from get_user, and retry the futex from
> futex_wake_op().
> 
> This would however also mean that there is no need for a spinlock
> at all, atomicity is already implied by pagefault_disable() here
> because you are running on a UP kernel and pagefault_disable() also
> means there is no preemption.
> 
> If this understanding is right, we can probably just merge the
> m68k implementation into the asm-generic version, as that does
> exactly that, and just isn't SMP safe. I'm still unsure whether
> I'm missing something here though, as everything else seems to
> do this in assembly, even for non-SMP machines that could use
> the trivial method that m68k has.

For UP relying of pagefault disable should be good enough indeed. I
guess the asm for the other UP stuff results from looking at
architectures or copying from architectures which did this in ASM :)

Thanks,

	tglx
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