linux-kernel - Re: [PATCH v3 0/5] MIPS: Add per-cpu IRQ stack

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Date:   Sun, 15 Jan 2017 14:48:48 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     James Hogan <james.hogan@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Matt Redfearn <matt.redfearn@...tec.com>,
        linux-mips@...ux-mips.org, Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marcin Nowakowski <marcin.nowakowski@...tec.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Petr Mladek <pmladek@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        Paul Burton <paul.burton@...tec.com>,
        Jiri Slaby <jslaby@...e.cz>,
        "Maciej W. Rozycki" <macro@...tec.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 0/5] MIPS: Add per-cpu IRQ stack

On Sun, Jan 15, 2017 at 02:39:49PM +0100, Jason A. Donenfeld wrote:
> Hi James,
> 
> On Fri, Jan 13, 2017 at 10:49 AM, James Hogan <james.hogan@...tec.com> wrote:
> > Its quite a significant change/feature, especially in terms of potential
> > for further breakage. I don't think its really stable material to be
> > honest.  It sounds bad if the kernel stack requirement can be made
> > arbitrarily large by stacking too many drivers.
> 
> Indeed I believe this is the case. If, say, a kthread is already using
> a bit of stack, and then a softirq chain of stacked virtual network
> drivers is called, the stack can be busted.
> 
> > Is there a simpler fix/workaround for the issue that would satisfy
> > stable kernel users until they can upgrade to a kernel with irqstacks?
> 
> The simplest solution is probably just not stacking tons of network
> drivers. For my own out-of-tree curve25519-donna code that's in
> OpenWRT and uses a fair amount of stack, I just kmalloc on MIPS but
> not on x86, so in terms of my own stuff there's already a workaround
> in place. But this still doesn't solve things for users who have some
> interesting networking requirements and stack a few drivers.
> 
> Unfortunately, most folks are only testing stuff on ARM and x86, which
> already have the separate IRQ stacks, so they aren't hitting crashes.
> 
> So, in the end, I'm not quite sure. On the one hand, this fixes an
> actual problem and it'd be nice to see stable kernels have the fix. On
> the other hand, this is a rather big change. I don't know how to
> assess it, but I've copied Greg on this email, who certainly has
> better judgement about this than me.

How many patches is the irqstacks "feature" for MIPS?  What kernel was
it released in?  Have any git commit ids I can look at?

thanks,

greg k-h