Date:   Mon, 3 Dec 2018 16:01:40 +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: [PATCH v6 3/4] kgdb: Don't round up a CPU that failed rounding
 up before

On Tue, Nov 27, 2018 at 09:38:38AM -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>


> ---
> 
> 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.rc0.387.gc7a69e6b6c-goog
> 

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