linux-kernel - Re: USB mass storage and ARM cache coherency

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Date:	Sun, 28 Feb 2010 23:17:14 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>,
	linux-usb@...r.kernel.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Mankad, Maulik Ojas" <x0082077@...com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Ming Lei <tom.leiming@...il.com>,
	Sebastian Siewior <bigeasy@...utronix.de>,
	Oliver Neukum <oliver@...kum.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Pavel Machek <pavel@....cz>, Greg KH <greg@...ah.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: USB mass storage and ARM cache coherency

On Fri, 2010-02-26 at 21:49 +0000, Benjamin Herrenschmidt wrote:
> > > > On ARM11MPCore we flush the caches in flush_dcache_page() because the
> > > > cache maintenance operations weren't visible to the other CPUs.
> > >
> > > I'm not even sure that's going to be 100% correct. Don't you also need
> > > to flush the remote icaches when you are dealing with instructions (such
> > > as swap) anyways ?
> >
> > I don't think we tried swap but for pages that have been mapped for the
> > first time, the I-cache would be clean.
> >
> > At mm switching, if a thread
> > migrates to a new CPU we invalidate the cache at that point.
> 
> That sounds fragile. What about a multithread app with one thread on
> each core hitting the pages at the same time ? Sounds racy to me...

Interestingly, until commit 826cbdaff29 (< 2 years ago), we didn't have
any I-cache flushing in update_mmu_cache() and it was working fine. I
added it for correctness reasons rather than to fix something. My theory
is that it was working because a page cache page tends to keep the same
physical address, especially if we don't swap pages, and a 16KB PIPT
cache cannot hold enough lines to show any issues (lines are replaced
frequently).

I suspect that's one of the reasons why only invalidating the whole
I-cache when switching the mm to a new CPU seems to suffice. Once we
enable some form of swapping, it may show the problem.

-- 
Catalin

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