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Message-ID: <20181219165538.dbrymi2y5jno2ih7@holly.lan>
Date: Wed, 19 Dec 2018 16:55:38 +0000
From: Daniel Thompson <daniel.thompson@...aro.org>
To: Douglas Anderson <dianders@...omium.org>
Cc: Jason Wessel <jason.wessel@...driver.com>,
Will Deacon <will.deacon@....com>,
kgdb-bugreport@...ts.sourceforge.net,
Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org
Subject: Re: [REPOST PATCH v6 3/4] kgdb: Don't round up a CPU that failed
rounding up before
On Tue, Dec 04, 2018 at 07:38:27PM -0800, Douglas Anderson wrote:
> If we're using the default implementation of kgdb_roundup_cpus() that
> uses smp_call_function_single_async() we can end up hanging
> kgdb_roundup_cpus() if we try to round up a CPU that failed to round
> up before.
>
> Specifically smp_call_function_single_async() will try to wait on the
> csd lock for the CPU that we're trying to round up. If the previous
> round up never finished then that lock could still be held and we'll
> just sit there hanging.
>
> There's not a lot of use trying to round up a CPU that failed to round
> up before. Let's keep a flag that indicates whether the CPU started
> but didn't finish to round up before. If we see that flag set then
> we'll skip the next round up.
>
> In general we have a few goals here:
> - We never want to end up calling smp_call_function_single_async()
> when the csd is still locked. This is accomplished because
> flush_smp_call_function_queue() unlocks the csd _before_ invoking
> the callback. That means that when kgdb_nmicallback() runs we know
> for sure the the csd is no longer locked. Thus when we set
> "rounding_up = false" we know for sure that the csd is unlocked.
> - If there are no timeouts rounding up we should never skip a round
> up.
>
> NOTE #1: In general trying to continue running after failing to round
> up CPUs doesn't appear to be supported in the debugger. When I
> simulate this I find that kdb reports "Catastrophic error detected"
> when I try to continue. I can overrule and continue anyway, but it
> should be noted that we may be entering the land of dragons here.
> Possibly the "Catastrophic error detected" was added _because_ of the
> future failure to round up, but even so this is an area of the code
> that hasn't been strongly tested.
>
> NOTE #2: I did a bit of testing before and after this change. I
> introduced a 10 second hang in the kernel while holding a spinlock
> that I could invoke on a certain CPU with 'taskset -c 3 cat /sys/...".
>
> Before this change if I did:
> - Invoke hang
> - Enter debugger
> - g (which warns about Catastrophic error, g again to go anyway)
> - g
> - Enter debugger
>
> ...I'd hang the rest of the 10 seconds without getting a debugger
> prompt. After this change I end up in the debugger the 2nd time after
> only 1 second with the standard warning about 'Timed out waiting for
> secondary CPUs.'
>
> I'll also note that once the CPU finished waiting I could actually
> debug it (aka "btc" worked)
>
> I won't promise that everything works perfectly if the errant CPU
> comes back at just the wrong time (like as we're entering or exiting
> the debugger) but it certainly seems like an improvement.
>
> NOTE #3: setting 'kgdb_info[cpu].rounding_up = false' is in
> kgdb_nmicallback() instead of kgdb_call_nmi_hook() because some
> implementations override kgdb_call_nmi_hook(). It shouldn't hurt to
> have it in kgdb_nmicallback() in any case.
>
> NOTE #4: this logic is really only needed because there is no API call
> like "smp_try_call_function_single_async()" or "smp_csd_is_locked()".
> If such an API existed then we'd use it instead, but it seemed a bit
> much to add an API like this just for kgdb.
>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Acked-by: Daniel Thompson <daniel.thompson@...aro.org>
Applied! Thanks.
> ---
>
> Changes in v6:
> - Moved smp_call_function_single_async() error check to patch 3.
>
> Changes in v5: None
> Changes in v4:
> - Removed smp_mb() calls.
>
> Changes in v3:
> - Don't round up a CPU that failed rounding up before new for v3.
>
> Changes in v2: None
>
> kernel/debug/debug_core.c | 20 +++++++++++++++++++-
> kernel/debug/debug_core.h | 1 +
> 2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> index 10db2833a423..1fb8b239e567 100644
> --- a/kernel/debug/debug_core.c
> +++ b/kernel/debug/debug_core.c
> @@ -247,6 +247,7 @@ void __weak kgdb_roundup_cpus(void)
> call_single_data_t *csd;
> int this_cpu = raw_smp_processor_id();
> int cpu;
> + int ret;
>
> for_each_online_cpu(cpu) {
> /* No need to roundup ourselves */
> @@ -254,8 +255,23 @@ void __weak kgdb_roundup_cpus(void)
> continue;
>
> csd = &per_cpu(kgdb_roundup_csd, cpu);
> +
> + /*
> + * If it didn't round up last time, don't try again
> + * since smp_call_function_single_async() will block.
> + *
> + * If rounding_up is false then we know that the
> + * previous call must have at least started and that
> + * means smp_call_function_single_async() won't block.
> + */
> + if (kgdb_info[cpu].rounding_up)
> + continue;
> + kgdb_info[cpu].rounding_up = true;
> +
> csd->func = kgdb_call_nmi_hook;
> - smp_call_function_single_async(cpu, csd);
> + ret = smp_call_function_single_async(cpu, csd);
> + if (ret)
> + kgdb_info[cpu].rounding_up = false;
> }
> }
>
> @@ -788,6 +804,8 @@ int kgdb_nmicallback(int cpu, void *regs)
> struct kgdb_state kgdb_var;
> struct kgdb_state *ks = &kgdb_var;
>
> + kgdb_info[cpu].rounding_up = false;
> +
> memset(ks, 0, sizeof(struct kgdb_state));
> ks->cpu = cpu;
> ks->linux_regs = regs;
> diff --git a/kernel/debug/debug_core.h b/kernel/debug/debug_core.h
> index 127d9bc49fb4..b4a7c326d546 100644
> --- a/kernel/debug/debug_core.h
> +++ b/kernel/debug/debug_core.h
> @@ -42,6 +42,7 @@ struct debuggerinfo_struct {
> int ret_state;
> int irq_depth;
> int enter_kgdb;
> + bool rounding_up;
> };
>
> extern struct debuggerinfo_struct kgdb_info[];
> --
> 2.20.0.rc1.387.gf8505762e3-goog
>
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