linux-kernel - Re: [PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call

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Message-ID: <20181103104503.eftn5btx7otgufro@holly.lan>
Date:   Sat, 3 Nov 2018 10:45:03 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        kgdb-bugreport@...ts.sourceforge.net,
        Peter Zijlstra <peterz@...radead.org>,
        linux-mips@...ux-mips.org, linux-sh@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Catalin Marinas <catalin.marinas@....com>, jhogan@...nel.org,
        linux-hexagon@...r.kernel.org, Vineet Gupta <vgupta@...opsys.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>, dalias@...c.org,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-snps-arc@...ts.infradead.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Will Deacon <will.deacon@....com>, paulus@...ba.org,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        christophe.leroy@....fr, mpe@...erman.id.au, paul.burton@...s.com,
        LKML <linux-kernel@...r.kernel.org>, rkuo@...eaurora.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used
 smp_call_function()

On Wed, Oct 31, 2018 at 02:41:14PM -0700, Doug Anderson wrote:
> > As mentioned in another part of the thread we can also add robustness
> > by skipping a cpu where csd->flags != 0 (and adding an appropriately
> > large comment regarding why). Doing the check directly is abusing
> > internal knowledge that smp.c normally keeps to itself so an accessor
> > of some kind would be needed.
> 
> Sure.  I could add smp_async_func_finished() that just looked like:
> 
> int smp_async_func_finished(call_single_data_t *csd)
> {
>   return !(csd->flags & CSD_FLAG_LOCK);
> }
> 
> My understanding of all the mutual exclusion / memory barrier concepts
> employed by smp.c is pretty weak, though.  I'm hoping that it's safe
> to just access the structure and check the bit directly.
> 
> ...but do you think adding a generic accessor like this is better than
> just keeping track of this in kgdb directly?  I could avoid the
> accessor by adding a "rounding_up" member to "struct
> debuggerinfo_struct" and doing something like this in roundup:
> 
>   /* If it didn't round up last time, don't try again */
>   if (kgdb_info[cpu].rounding_up)
>     continue
> 
>   kgdb_info[cpu].rounding_up = true
>   smp_call_function_single_async(cpu, csd);
> 
> ...and then in kgdb_nmicallback() I could just add:
> 
>   kgdb_info[cpu].rounding_up = false
> 
> In that case we're not adding a generic accessor to smp.c that most
> people should never use.

Whilst it is very tempting to make a sarcastic reply here ("Of course! What
kgdb really needs is yet another set of condition variables") I can't
because I actually agree with the proposal. I don't really want kgdb to
be too "special" especially when it doesn't need to be.

Only thing to note is that rounding_up will not be manipulated within a
common spin lock so you might have to invest a bit of thought to make
sure any races between the master and slave as the slave CPU clears the
flag are benign.


Daniel.