Re: arm64: pstore: printk causing hang during boot in __memcpy_toio with pstore enabled

[Guenter Roeck <linux@roeck-us.net>]

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@roeck-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