linux-kernel - Re: [PATCH v3 1/7] dump_stack: Support adding to the dump stack arch description

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Message-ID: <20190211075524.GA26690@jagdpanzerIV>
Date:   Mon, 11 Feb 2019 16:55:24 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>, linuxppc-dev@...abs.org,
        akpm@...ux-foundation.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        dyoung@...hat.com, sergey.senozhatsky@...il.com, pmladek@...e.com
Subject: Re: [PATCH v3 1/7] dump_stack: Support adding to the dump stack arch
 description

On (02/08/19 13:55), Steven Rostedt wrote:
[..]
> > +	if (len) {
> > +		/*
> > +		 * Order the stores above in vsnprintf() vs the store of the
> > +		 * space below which joins the two strings. Note this doesn't
> > +		 * make the code truly race free because there is no barrier on
> > +		 * the read side. ie. Another CPU might load the uninitialised
> > +		 * tail of the buffer first and then the space below (rather
> > +		 * than the NULL that was there previously), and so print the
> > +		 * uninitialised tail. But the whole string lives in BSS so in
> > +		 * practice it should just see NULLs.
> > +		 */
> > +		smp_wmb();
> 
> This shows me that this can be called at a time when more than one CPU is
> active. What happens if we have two CPUs calling dump_stack_add_arch_desc() at
> the same time? Can't that corrupt the dump_stack_arch_desc_str?

Can overwrite part of it, I guess (but it seems that Michael
is OK with this). The string is still NULL terminated.

The worst case scenario I can think of is not the one when
two CPUs call dump_stack_add_arch_desc(), but when CPUA calls
dump_stack_add_arch_desc() to append some data and at the
same time CPUB calls dump_stack_set_arch_desc() and simply
overwrites dump_stack_arch_desc_str. Not sure if this is
critical (or possible).

	-ss