arm_syscall cacheflush breakage on VIPT platforms

[imre.deak@nokia.com (Imre Deak)]

On Mon, Sep 28, 2009 at 03:19:26PM +0200, ext Jamie Lokier wrote:
> Imre Deak wrote:
> > On Mon, Sep 28, 2009 at 02:49:22PM +0200, ext Jamie Lokier wrote:
> > > Imre Deak wrote:
> > > > Hi,
> > > > 
> > > > the following test app will cause an unhandled kernel paging request
> > > > on VIPT platforms. The triggering condition is the mmap_sem held by
> > > > thread_func while the main thread performs cache flushing.
> > > > 
> > > > Since the likelihood of this to trigger is relatively low, a patch will
> > > > follow that makes similar bugs more visible.
> > > 
> > > I would expect the likelihood of triggering would be higher for at
> > > least one of Java, Mono, Parrot or any of the modern Javascript
> > > engines.
> > 
> > True, the above statement is only valid for certain use patterns. I was
> > mainly interested in applications that do user range cache flushing as
> > part of their DMA requests and they didn't have threads with frequent
> > syscalls that required mmap_sem, so the problem remained hidden for a
> > long time.
> 
> Aieee.  Is sys_cacheflush architecturally the Right Way to do DMA to
> userspace, or is it just luck that it happens to work?

No, it's not sys_cacheflush but using dma_cache_maint for user range.
And yes I know that at the moment it's not the Right Way to use it on
a user range, but the alternative of flushing each page separately is
just prohibitively slow.

Hopefully by adding the necessary fixups for the cache ops (and taking
mmap_sem) will make this an ok thing to do. An alternative is to
mlock the range so no faults are triggered for it, but that's again a
not-supported-thing to do from a driver.

> Does that include O_DIRECT regular file I/O as used by databases on
> these ARMs?  (Nobody ever gives a straight answer)

Not in my case, it's just a regular anonymous mapping pinned with
get_user_pages.

--Imre