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From: Daniel Thompson <daniel.thompson@linaro.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Douglas Anderson <dianders@chromium.org>,
	Jason Wessel <jason.wessel@windriver.com>,
	kgdb-bugreport@lists.sourceforge.net, linux-mips@linux-mips.org,
	linux-sh@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Hogan <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>,
	Rich Felker <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>,
	Paul Mackerras <paulus@samba.org>,
	Russell King <linux@armlinux.org.uk>,
	linux-arm-kernel@lists.infradead.org,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Paul Burton <paul.burton@mips.com>,
	linux-kernel@vger.kernel.org, Richard Kuo <rkuo@codeaurora.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()
Date: Wed, 31 Oct 2018 17:01:36 +0000	[thread overview]
Message-ID: <20181031170136.s3ids6tm4rxxlpma@holly.lan> (raw)
In-Reply-To: <20181031134926.GB13237@hirez.programming.kicks-ass.net>

On Wed, Oct 31, 2018 at 02:49:26PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 30, 2018 at 03:18:43PM -0700, Douglas Anderson wrote:
> > Looking closely at it, it seems like a really bad idea to be calling
> > local_irq_enable() in kgdb_roundup_cpus().  If nothing else that seems
> > like it could violate spinlock semantics and cause a deadlock.
> > 
> > Instead, let's use a private csd alongside
> > smp_call_function_single_async() to round up the other CPUs.  Using
> > smp_call_function_single_async() doesn't require interrupts to be
> > enabled so we can remove the offending bit of code.
> 
> You might want to mention that the only reason this isn't a deadlock
> itself is because there is a timeout on waiting for the slaves to
> check-in.

dbg_master_lock must be owned to call kgdb_roundup_cpus() so
the calls to smp_call_function_single_async() should never deadlock the
calling CPU unless there has been a previous failure to round up (e.g.
cores that cannot react to the round up signal).

When there is a failure to round up when we resume, there is a window (before
whatever locks that prevented the IPI being serviced are released) during which
the system will deadlock if the debugger is re entered.

I don't think there is any point trying to round up a CPU that did not
previously respond... it should still have an IPI pending. The deadlock
can be eliminated by getting the round up code to avoid CPUs whose csd->flags
are non-zero either by checking the flag in the kgdb code or adding something
like smp_trycall_function_single_async().


Daniel.

  reply	other threads:[~2018-10-31 17:01 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 [this message]
2018-10-31 18:40   ` Daniel Thompson
2018-10-31 21:41     ` Doug Anderson
2018-11-03 10:45       ` Daniel Thompson
2018-11-07  1:04         ` Doug Anderson

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