linux-kernel - Re: arm64: pstore: printk causing hang during boot in __memcpy

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Message-ID: <20180801172549.GA16774@roeck-us.net>
Date:   Wed, 1 Aug 2018 10:25:49 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Ivid Suvarna <ivid.suvarna@...il.com>
Cc:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Baron <jbaron@...hat.com>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        Joe Perches <joe@...ches.com>, linux-arm-msm@...r.kernel.org
Subject: Re: arm64: pstore: printk causing hang during boot in __memcpy_toio
 with pstore enabled

On Wed, Aug 01, 2018 at 10:46:06PM +0530, Ivid Suvarna wrote:
> On Wed, Aug 1, 2018 at 6:49 PM, Guenter Roeck <linux@...ck-us.net> wrote:
> > On 08/01/2018 05:35 AM, Ivid Suvarna wrote:
> >>
> >> Hi,
> >>
> >> When pstore is enabled and a *pr_info(any printk) in
> >> __memcpy_toio(arch/arm64/kernel/io.c)* is added, kernel wont boot and
> >> just hangs.
> >>
> >> The path where __memcpy_toio is called is below:
> >>
> >> ->persistent_ram_update
> >>    -> memcpy_toio
> >>      -> __memcpy_toio
> >>
> >> I tried with trace_printk and kernel boots fine. I understand that
> >> printk has overhead, but is this expected when we use some printk
> >> statement in __memcpy_toio?
> >>
> >
> > I think the problem may be that the printk() output is copied to pstore.
> > Since pstore calls memcpy_toio(), you get a nice recursion if you add a
> > printk() call to it.
> >
> 
> Is there any solution to this other than not adding printk :p

Well, disabling pstore would help. You could also use a trace function,
as you already found out, or you could use gdb for debugging. Since
this is obviously a debug image, you should be willing to accept some
limitations/restrictions.

Guenter