Subject: [Kgdb-bugreport] [PATCH] ARM: change definition of cpu_relax() for ARM11MPCore

From: [email-masked] (DDD)
Date: Thu, 11 Mar 2010 10:47:34 +0800

Jason Wessel wrote:
> Will Deacon wrote:
> 
>>> Clearly, kgdb is using atomic_set()/atomic_read() in a way which does not
>>> match this documentation - it's certainly missing the barriers as required
>>> by the above quoted paragraphs.
>>>
>>> Let me repeat: atomic_set() and atomic_read() are NOT atomic.  There's
>>> nothing atomic about them.  All they do is provide a pair of accessors
>>> to the underlying value in the atomic type.  They are no different to
>>> declaring a volatile int and reading/writing it directly.
> 
> 
> Clearly the docs state this.
> 
>> Indeed. I'm not familiar enough with KGDB internals to dive in and look at all the
>> potential barrier conflicts, so I'll submit a patch that addresses the one that's
>> bitten me so far. Maybe it will motivate somebody else to take a closer look!
>>
> 
> 
> Do you think you can try the patch below?
> 
> It seems we might not need to change to using the atomic_add_return(0,...) because using the atomic_inc() and atomic_dec() will end up using the memory barriers.
>

Hi Jason,

Maybe we should initial the atomic_t variable before we using such as 
atomic_inc/dec() directly.

Dongdong.

--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -227,6 +227,17 @@ kgdb_post_primary_code(struct pt_regs *regs, int 
e_vector, int err_code)
         return;
  }

+static void kgdb_initial_atomic_var()
+{
+       int i;
+       for (i = NR_CPUS-1; i >= 0; i--) {
+               atomic_set(&passive_cpu_wait[i], 0);
+               atomic_set(&cpu_in_kgdb[i], 0);
+       }
+
+       atomic_set(&kgdb_setting_breakpoint, 0);
+}
+
  /**
   *     kgdb_disable_hw_debug - Disable hardware debugging while we in 
kgdb.
   *     @regs: Current &struct pt_regs.
@@ -1619,6 +1630,7 @@ static void kgdb_register_callbacks(void)
  {
         if (!kgdb_io_module_registered) {
                 kgdb_io_module_registered = 1;
+               kgdb_initial_atomic_var();
                 kgdb_arch_init();
  #ifdef CONFIG_MAGIC_SYSRQ
                 register_sysrq_key('g', &sysrq_gdb_op);

> I would certainly rather fix kgdb vs mucking with the internals of cpu_relax().
> 
> 
> Jason.
> 
> --- a/kernel/kgdb.c
> +++ b/kernel/kgdb.c
> @@ -580,14 +580,13 @@ static void kgdb_wait(struct pt_regs *re
>  	 * Make sure the above info reaches the primary CPU before
>  	 * our cpu_in_kgdb[] flag setting does:
>  	 */
> -	smp_wmb();
> -	atomic_set(&cpu_in_kgdb[cpu], 1);
> +	atomic_inc(&cpu_in_kgdb[cpu]);
>  
>  	/* Disable any cpu specific hw breakpoints */
>  	kgdb_disable_hw_debug(regs);
>  
>  	/* Wait till primary CPU is done with debugging */
> -	while (atomic_read(&passive_cpu_wait[cpu]))
> +	while (atomic_add_return(0, &passive_cpu_wait[cpu]))
>  		cpu_relax();
>  
>  	kgdb_info[cpu].debuggerinfo = NULL;
> @@ -598,7 +597,7 @@ static void kgdb_wait(struct pt_regs *re
>  		arch_kgdb_ops.correct_hw_break();
>  
>  	/* Signal the primary CPU that we are done: */
> -	atomic_set(&cpu_in_kgdb[cpu], 0);
> +	atomic_dec(&cpu_in_kgdb[cpu]);
>  	touch_softlockup_watchdog_sync();
>  	clocksource_touch_watchdog();
>  	local_irq_restore(flags);
> @@ -1493,7 +1492,7 @@ acquirelock:
>  	 * spin_lock code is good enough as a barrier so we don't
>  	 * need one here:
>  	 */
> -	atomic_set(&cpu_in_kgdb[ks->cpu], 1);
> +	atomic_inc(&cpu_in_kgdb[ks->cpu]);
>  
>  #ifdef CONFIG_SMP
>  	/* Signal the other CPUs to enter kgdb_wait() */
> @@ -1505,7 +1504,7 @@ acquirelock:
>  	 * Wait for the other CPUs to be notified and be waiting for us:
>  	 */
>  	for_each_online_cpu(i) {
> -		while (!atomic_read(&cpu_in_kgdb[i]))
> +		while (!atomic_add_return(0, &cpu_in_kgdb[i]))
>  			cpu_relax();
>  	}
>  
> @@ -1528,7 +1527,7 @@ acquirelock:
>  
>  	kgdb_info[ks->cpu].debuggerinfo = NULL;
>  	kgdb_info[ks->cpu].task = NULL;
> -	atomic_set(&cpu_in_kgdb[ks->cpu], 0);
> +	atomic_dec(&cpu_in_kgdb[ks->cpu]);
>  
>  	if (!kgdb_single_step) {
>  		for (i = NR_CPUS-1; i >= 0; i--)
> @@ -1538,7 +1537,7 @@ acquirelock:
>  		 * from the debugger.
>  		 */
>  		for_each_online_cpu(i) {
> -			while (atomic_read(&cpu_in_kgdb[i]))
> +			while (atomic_add_return(0, &cpu_in_kgdb[i]))
>  				cpu_relax();
>  		}
>  	}
> @@ -1742,11 +1741,11 @@ EXPORT_SYMBOL_GPL(kgdb_unregister_io_mod
>   */
>  void kgdb_breakpoint(void)
>  {
> -	atomic_set(&kgdb_setting_breakpoint, 1);
> +	atomic_inc(&kgdb_setting_breakpoint);
>  	wmb(); /* Sync point before breakpoint */
>  	arch_kgdb_breakpoint();
>  	wmb(); /* Sync point after breakpoint */
> -	atomic_set(&kgdb_setting_breakpoint, 0);
> +	atomic_dec(&kgdb_setting_breakpoint);
>  }
>  EXPORT_SYMBOL_GPL(kgdb_breakpoint);
>  
> 
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