Re: [PATCH/RFC] Futex mmap_sem deadlock

Olof Johansson
2005-02-23T18:42:14Z

On Wed, Feb 23, 2005 at 06:22:04PM +0000, Jamie Lokier wrote:
> Olof Johansson wrote:
> > On Wed, Feb 23, 2005 at 07:54:06AM -0800, Linus Torvalds wrote:
> >
> > > > Otherwise, a preempt attempt in get_user would not be seen
> > > > until some future preempt_enable was executed.
> > >
> > > True. I guess we should have a "preempt_check_resched()" there too. That's
> > > what "kunmap_atomic()" does too (which is what we rely on in the other
> > > case we do this..)
> >
> > Ok, this is getting complex enough to warrant get_user_inatomic(),
> > which means adding it to every arch's uaccess.h.
> >
> > Below patch does so. Unfortunately I don't have a Viro setup with cross
> > compilers for nearly every arch, so I can't make sure it doesn't break
> > anything. But since I pasted the same code everywhere it shouldn't.
>
> My turn to say uglee.

Yeah, I wasn't entirely happy about it, but it seems that suggestions
are coming in on how to do it better. :-)

> Firstly, get_user_inatomic is the wrong name.
>
> "inatomic" in __copy_from_user_inatomic means it's called inside a
> non-premptable region (in atomic...).
>
> Your macro get_user_inatomic is _not_ called inside a
> non-preemptable region, so it shouldn't be called "inatomic".
>
> (A better name is get_user_no_paging).
>
> Secondly, does this _one_ use (it's not likely to be used elsewhere)
> justify copying & pasting the same code into every asm-*/uaccess,
> especially when the code is not in any way arch-specific?

Arjan suggested creating a linux/uaccess.h that includes asm/uaccess.h,
and start moving the users over since that's where the trend is moving
anyway (avoiding including asm/* from common code). futex.c would be
the first user, and could be followed by more later as a janitorial
patch. That'd mean only one addition of the common function instead of
having to add it to every arch.

> I suggest putting it into futex.c, and make it an inline function
> which takes "u32 __user *".

Sure, for now that's good enough. Above janitorial work could be done
later, if more users get introduced.

-Olof