Re: [PATCH v6 2/4] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()

[Daniel Thompson]

On Tue, Nov 27, 2018 at 09:38:37AM -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@xxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
> Changes in v6:
> - Moved smp_call_function_single_async() error check to patch 3.
> 
> 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()
> 
> Changes in v4: None
> Changes in v3:
> - No separate init call.
> - Don't round up the CPU that is doing the rounding up.
> - Add "#ifdef CONFIG_SMP" to match the rest of the file.
> - Updated desc saying we don't solve the "failed to roundup" case.
> - Document the ignored parameter.
> 
> Changes in v2:
> - Removing irq flags separated from fixing lockdep splat.
> - Don't use smp_call_function (Daniel).
> 
>  arch/arc/kernel/kgdb.c     | 10 ++--------
>  arch/arm/kernel/kgdb.c     | 12 -----------
>  arch/arm64/kernel/kgdb.c   | 12 -----------
>  arch/hexagon/kernel/kgdb.c | 27 -------------------------
>  arch/mips/kernel/kgdb.c    |  9 +--------
>  arch/powerpc/kernel/kgdb.c |  4 ++--
>  arch/sh/kernel/kgdb.c      | 12 -----------

Please could you re-send with the arch maintainers for these platforms
included in the Cc: of the patch description rather than just in the
e-mail header.

I'd like to make sure arch maintainers have a chance to opt out if they
want to (it's a weak symbol so an arch could chose not to use it).

Otherwise I shall start testing shortly!


Daniel.


>  include/linux/kgdb.h       | 15 ++++++++++++--
>  kernel/debug/debug_core.c  | 41 ++++++++++++++++++++++++++++++++++++++
>  9 files changed, 59 insertions(+), 83 deletions(-)
> 
> diff --git a/arch/arc/kernel/kgdb.c b/arch/arc/kernel/kgdb.c
> index 0932851028e0..68d9fe4b5aa7 100644
> --- a/arch/arc/kernel/kgdb.c
> +++ b/arch/arc/kernel/kgdb.c
> @@ -192,18 +192,12 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long ip)
>  	instruction_pointer(regs) = ip;
>  }
>  
> -static void kgdb_call_nmi_hook(void *ignored)
> +void kgdb_call_nmi_hook(void *ignored)
>  {
> +	/* Default implementation passes get_irq_regs() but we don't */
>  	kgdb_nmicallback(raw_smp_processor_id(), NULL);
>  }
>  
> -void kgdb_roundup_cpus(void)
> -{
> -	local_irq_enable();
> -	smp_call_function(kgdb_call_nmi_hook, NULL, 0);
> -	local_irq_disable();
> -}
> -
>  struct kgdb_arch arch_kgdb_ops = {
>  	/* breakpoint instruction: TRAP_S 0x3 */
>  #ifdef CONFIG_CPU_BIG_ENDIAN
> diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
> index f21077b077be..d9a69e941463 100644
> --- a/arch/arm/kernel/kgdb.c
> +++ b/arch/arm/kernel/kgdb.c
> @@ -170,18 +170,6 @@ static struct undef_hook kgdb_compiled_brkpt_hook = {
>  	.fn			= kgdb_compiled_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();
> -}
> -
>  static int __kgdb_notify(struct die_args *args, unsigned long cmd)
>  {
>  	struct pt_regs *regs = args->regs;
> 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();
> -}
> -
>  static int __kgdb_notify(struct die_args *args, unsigned long cmd)
>  {
>  	struct pt_regs *regs = args->regs;
> diff --git a/arch/hexagon/kernel/kgdb.c b/arch/hexagon/kernel/kgdb.c
> index 012e0e230ac2..b95d12038a4e 100644
> --- a/arch/hexagon/kernel/kgdb.c
> +++ b/arch/hexagon/kernel/kgdb.c
> @@ -115,33 +115,6 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
>  	instruction_pointer(regs) = pc;
>  }
>  
> -#ifdef CONFIG_SMP
> -
> -/**
> - * kgdb_roundup_cpus - Get other CPUs into a holding pattern
> - *
> - * On SMP systems, we need to get the attention of the other CPUs
> - * and get them be in a known state.  This should do what is needed
> - * to get the other CPUs to call kgdb_wait(). Note that on some arches,
> - * the NMI approach is not used for rounding up all the CPUs. For example,
> - * in case of MIPS, smp_call_function() is used to roundup CPUs.
> - *
> - * On non-SMP systems, this is not called.
> - */
> -
> -static void hexagon_kgdb_nmi_hook(void *ignored)
> -{
> -	kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
> -}
> -
> -void kgdb_roundup_cpus(void)
> -{
> -	local_irq_enable();
> -	smp_call_function(hexagon_kgdb_nmi_hook, NULL, 0);
> -	local_irq_disable();
> -}
> -#endif
> -
>  
>  /*  Not yet working  */
>  void sleeping_thread_to_gdb_regs(unsigned long *gdb_regs,
> diff --git a/arch/mips/kernel/kgdb.c b/arch/mips/kernel/kgdb.c
> index 2b05effc17b4..42f057a6c215 100644
> --- a/arch/mips/kernel/kgdb.c
> +++ b/arch/mips/kernel/kgdb.c
> @@ -207,7 +207,7 @@ void arch_kgdb_breakpoint(void)
>  		".set\treorder");
>  }
>  
> -static void kgdb_call_nmi_hook(void *ignored)
> +void kgdb_call_nmi_hook(void *ignored)
>  {
>  	mm_segment_t old_fs;
>  
> @@ -219,13 +219,6 @@ static void kgdb_call_nmi_hook(void *ignored)
>  	set_fs(old_fs);
>  }
>  
> -void kgdb_roundup_cpus(void)
> -{
> -	local_irq_enable();
> -	smp_call_function(kgdb_call_nmi_hook, NULL, 0);
> -	local_irq_disable();
> -}
> -
>  static int compute_signal(int tt)
>  {
>  	struct hard_trap_info *ht;
> diff --git a/arch/powerpc/kernel/kgdb.c b/arch/powerpc/kernel/kgdb.c
> index b0e804844be0..b4ce54d73337 100644
> --- a/arch/powerpc/kernel/kgdb.c
> +++ b/arch/powerpc/kernel/kgdb.c
> @@ -117,7 +117,7 @@ int kgdb_skipexception(int exception, struct pt_regs *regs)
>  	return kgdb_isremovedbreak(regs->nip);
>  }
>  
> -static int kgdb_call_nmi_hook(struct pt_regs *regs)
> +static int kgdb_debugger_ipi(struct pt_regs *regs)
>  {
>  	kgdb_nmicallback(raw_smp_processor_id(), regs);
>  	return 0;
> @@ -502,7 +502,7 @@ int kgdb_arch_init(void)
>  	old__debugger_break_match = __debugger_break_match;
>  	old__debugger_fault_handler = __debugger_fault_handler;
>  
> -	__debugger_ipi = kgdb_call_nmi_hook;
> +	__debugger_ipi = kgdb_debugger_ipi;
>  	__debugger = kgdb_debugger;
>  	__debugger_bpt = kgdb_handle_breakpoint;
>  	__debugger_sstep = kgdb_singlestep;
> diff --git a/arch/sh/kernel/kgdb.c b/arch/sh/kernel/kgdb.c
> index cc57630f6bf2..14e012ad7c57 100644
> --- a/arch/sh/kernel/kgdb.c
> +++ b/arch/sh/kernel/kgdb.c
> @@ -314,18 +314,6 @@ BUILD_TRAP_HANDLER(singlestep)
>  	local_irq_restore(flags);
>  }
>  
> -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();
> -}
> -
>  static int __kgdb_notify(struct die_args *args, unsigned long cmd)
>  {
>  	int ret;
> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index 05e5b2eb0d32..24422865cd18 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -176,14 +176,25 @@ kgdb_arch_handle_exception(int vector, int signo, int err_code,
>  			   char *remcom_out_buffer,
>  			   struct pt_regs *regs);
>  
> +/**
> + *	kgdb_call_nmi_hook - Call kgdb_nmicallback() on the current CPU
> + *	@ignored: This parameter is only here to match the prototype.
> + *
> + *	If you're using the default implementation of kgdb_roundup_cpus()
> + *	this function will be called per CPU.  If you don't implement
> + *	kgdb_call_nmi_hook() a default will be used.
> + */
> +
> +extern void kgdb_call_nmi_hook(void *ignored);
> +
>  /**
>   *	kgdb_roundup_cpus - Get other CPUs into a holding pattern
>   *
>   *	On SMP systems, we need to get the attention of the other CPUs
>   *	and get them into a known state.  This should do what is needed
>   *	to get the other CPUs to call kgdb_wait(). Note that on some arches,
> - *	the NMI approach is not used for rounding up all the CPUs. For example,
> - *	in case of MIPS, smp_call_function() is used to roundup CPUs.
> + *	the NMI approach is not used for rounding up all the CPUs.  Normally
> + *	those architectures can just not implement this and get the default.
>   *
>   *	On non-SMP systems, this is not called.
>   */
> diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> index f3cadda45f07..10db2833a423 100644
> --- a/kernel/debug/debug_core.c
> +++ b/kernel/debug/debug_core.c
> @@ -55,6 +55,7 @@
>  #include <linux/mm.h>
>  #include <linux/vmacache.h>
>  #include <linux/rcupdate.h>
> +#include <linux/irq.h>
>  
>  #include <asm/cacheflush.h>
>  #include <asm/byteorder.h>
> @@ -220,6 +221,46 @@ int __weak kgdb_skipexception(int exception, struct pt_regs *regs)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_SMP
> +
> +/*
> + * Default (weak) implementation for kgdb_roundup_cpus
> + */
> +
> +static DEFINE_PER_CPU(call_single_data_t, kgdb_roundup_csd);
> +
> +void __weak kgdb_call_nmi_hook(void *ignored)
> +{
> +	/*
> +	 * NOTE: get_irq_regs() is supposed to get the registers from
> +	 * before the IPI interrupt happened and so is supposed to
> +	 * show where the processor was.  In some situations it's
> +	 * possible we might be called without an IPI, so it might be
> +	 * safer to figure out how to make kgdb_breakpoint() work
> +	 * properly here.
> +	 */
> +	kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
> +}
> +
> +void __weak kgdb_roundup_cpus(void)
> +{
> +	call_single_data_t *csd;
> +	int this_cpu = raw_smp_processor_id();
> +	int cpu;
> +
> +	for_each_online_cpu(cpu) {
> +		/* No need to roundup ourselves */
> +		if (cpu == this_cpu)
> +			continue;
> +
> +		csd = &per_cpu(kgdb_roundup_csd, cpu);
> +		csd->func = kgdb_call_nmi_hook;
> +		smp_call_function_single_async(cpu, csd);
> +	}
> +}
> +
> +#endif
> +
>  /*
>   * Some architectures need cache flushes when we set/clear a
>   * breakpoint:
> -- 
> 2.20.0.rc0.387.gc7a69e6b6c-goog
>