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From: Daniel Thompson <daniel.thompson@linaro.org>
To: Doug Anderson <dianders@chromium.org>
Cc: Jason Wessel <jason.wessel@windriver.com>,
	kgdb-bugreport@lists.sourceforge.net,
	Peter Zijlstra <peterz@infradead.org>,
	linux-mips@linux-mips.org, linux-sh@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	jhogan@kernel.org, linux-hexagon@vger.kernel.org,
	Vineet Gupta <vgupta@synopsys.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	dalias@libc.org, Ralf Baechle <ralf@linux-mips.org>,
	linux-snps-arc@lists.infradead.org,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Will Deacon <will.deacon@arm.com>,
	paulus@samba.org,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	christophe.leroy@c-s.fr, mpe@ellerman.id.au,
	paul.burton@mips.com, LKML <linux-kernel@vger.kernel.org>,
	rkuo@codeaurora.org, linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()
Date: Sat, 3 Nov 2018 10:45:03 +0000	[thread overview]
Message-ID: <20181103104503.eftn5btx7otgufro@holly.lan> (raw)
In-Reply-To: <CAD=FV=V1eHo7Wz31DTMMNi394qwEaESTxJCYVE60Q7hpDEqRmQ@mail.gmail.com>

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.

  reply	other threads:[~2018-11-03 10:45 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-30 22:18 [PATCH v2 0/2] kgdb: Fix kgdb_roundup_cpus() Douglas Anderson
2018-10-30 22:18 ` [PATCH v2 1/2] kgdb: Remove irq flags from roundup Douglas Anderson
2018-10-30 22:18 ` [PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function() Douglas Anderson
2018-10-31  1:51   ` kbuild test robot
2018-10-31 13:49   ` Peter Zijlstra
2018-10-31 17:01     ` Daniel Thompson
2018-10-31 18:40   ` Daniel Thompson
2018-10-31 21:41     ` Doug Anderson
2018-11-03 10:45       ` Daniel Thompson [this message]
2018-11-07  1:04         ` Doug Anderson

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