From: daniel.thompson@linaro.org (Daniel Thompson)
To: linux-arm-kernel@lists.infradead.org
Subject: [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.
next prev parent 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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