From: Will Deacon <will.deacon@arm.com>
To: Douglas Anderson <dianders@chromium.org>
Cc: Jason Wessel <jason.wessel@windriver.com>,
Daniel Thompson <daniel.thompson@linaro.org>,
kgdb-bugreport@lists.sourceforge.net,
Peter Zijlstra <peterz@infradead.org>,
linux-mips@linux-mips.org, linux-sh@vger.kernel.org,
linux-kernel@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
James Hogan <jhogan@kernel.org>,
linux-hexagon@vger.kernel.org, Vineet Gupta <vgupta@synopsys.com>,
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>,
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>,
Richard Kuo <rkuo@codeaurora.org>,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v5 2/4] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()
Date: Tue, 27 Nov 2018 19:30:05 +0000 [thread overview]
Message-ID: <20181127193005.GB5641@arm.com> (raw)
In-Reply-To: <20181127035133.225592-3-dianders@chromium.org>
On Mon, Nov 26, 2018 at 07:51:31PM -0800, Douglas Anderson wrote:
> When I had lockdep turned on and dropped into kgdb I got a nice splat
> on my system. Specifically it hit:
> DEBUG_LOCKS_WARN_ON(current->hardirq_context)
>
> Specifically it looked like this:
> sysrq: SysRq : DEBUG
> ------------[ cut here ]------------
> DEBUG_LOCKS_WARN_ON(current->hardirq_context)
> WARNING: CPU: 0 PID: 0 at .../kernel/locking/lockdep.c:2875 lockdep_hardirqs_on+0xf0/0x160
> CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.19.0 #27
> pstate: 604003c9 (nZCv DAIF +PAN -UAO)
> pc : lockdep_hardirqs_on+0xf0/0x160
> ...
> Call trace:
> lockdep_hardirqs_on+0xf0/0x160
> trace_hardirqs_on+0x188/0x1ac
> kgdb_roundup_cpus+0x14/0x3c
> kgdb_cpu_enter+0x53c/0x5cc
> kgdb_handle_exception+0x180/0x1d4
> kgdb_compiled_brk_fn+0x30/0x3c
> brk_handler+0x134/0x178
> do_debug_exception+0xfc/0x178
> el1_dbg+0x18/0x78
> kgdb_breakpoint+0x34/0x58
> sysrq_handle_dbg+0x54/0x5c
> __handle_sysrq+0x114/0x21c
> handle_sysrq+0x30/0x3c
> qcom_geni_serial_isr+0x2dc/0x30c
> ...
> ...
> irq event stamp: ...45
> hardirqs last enabled at (...44): [...] __do_softirq+0xd8/0x4e4
> hardirqs last disabled at (...45): [...] el1_irq+0x74/0x130
> softirqs last enabled at (...42): [...] _local_bh_enable+0x2c/0x34
> softirqs last disabled at (...43): [...] irq_exit+0xa8/0x100
> ---[ end trace adf21f830c46e638 ]---
>
> 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.
>
> In order to avoid duplicating this across all the architectures that
> use the default kgdb_roundup_cpus(), we'll add a "weak" implementation
> to debug_core.c.
>
> Looking at all the people who previously had copies of this code,
> there were a few variants. I've attempted to keep the variants
> working like they used to. Specifically:
> * For arch/arc we passed NULL to kgdb_nmicallback() instead of
> get_irq_regs().
> * For arch/mips there was a bit of extra code around
> kgdb_nmicallback()
>
> NOTE: In this patch we will still get into trouble if we try to round
> up a CPU that failed to round up before. We'll try to round it up
> again and potentially hang when we try to grab the csd lock. That's
> not new behavior but we'll still try to do better in a future patch.
>
> Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> Changes in v5:
> - Add a comment about get_irq_regs().
> - get_cpu() => raw_smp_processor_id() in kgdb_roundup_cpus().
> - for_each_cpu() => for_each_online_cpu()
> - Error check smp_call_function_single_async()
For the arm64 bit:
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index 12c339ff6e75..da880247c734 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -284,18 +284,6 @@ static struct step_hook kgdb_step_hook = {
> .fn = kgdb_step_brk_fn
> };
>
> -static void kgdb_call_nmi_hook(void *ignored)
> -{
> - kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
> -}
> -
> -void kgdb_roundup_cpus(void)
> -{
> - local_irq_enable();
> - smp_call_function(kgdb_call_nmi_hook, NULL, 0);
> - local_irq_disable();
> -}
> -
Acked-by: Will Deacon <will.deacon@arm.com>
Will
next prev parent reply other threads:[~2018-11-27 19:29 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-27 3:51 [PATCH v5 0/4] kgdb: Fix kgdb_roundup_cpus() Douglas Anderson
2018-11-27 3:51 ` [PATCH v5 1/4] kgdb: Remove irq flags from roundup Douglas Anderson
2018-11-27 3:51 ` [PATCH v5 2/4] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function() Douglas Anderson
2018-11-27 5:38 ` kbuild test robot
2018-11-27 17:39 ` Doug Anderson
2018-11-27 19:30 ` Will Deacon [this message]
2018-11-27 3:51 ` [PATCH v5 3/4] kgdb: Don't round up a CPU that failed rounding up before Douglas Anderson
2018-11-27 3:51 ` [PATCH v5 4/4] kdb: Don't back trace on a cpu that didn't round up Douglas Anderson
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