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Date:   Wed, 31 Oct 2018 17:01:36 +0000
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()
Message-ID: <20181031170136.s3ids6tm4rxxlpma@holly.lan>
References: <20181030221843.121254-1-dianders@chromium.org>
 <20181030221843.121254-3-dianders@chromium.org>
 <20181031134926.GB13237@hirez.programming.kicks-ass.net>
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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.