[PATCH v4 3/4] AArch64: KGDB: Add step debugging support

[Vijay Kilari]

On Mon, Nov 11, 2013 at 8:30 PM, Will Deacon <will.deacon at arm.com> wrote:
> On Mon, Nov 11, 2013 at 11:09:46AM +0000, Vijay Kilari wrote:
>> On Fri, Nov 8, 2013 at 7:48 PM, Will Deacon <will.deacon at arm.com> wrote:
>> > On Tue, Nov 05, 2013 at 08:45:45AM +0000, vijay.kilari at gmail.com wrote:
>> >> @@ -153,6 +166,8 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>> >>                * Packet D (Detach), k (kill). No special handling
>> >>                * is required here
>> >>                */
>> >> +             atomic_set(&kgdb_cpu_doing_single_step, -1);
>> >> +             kgdb_single_step =  0;
>> >
>> > This looks really weird: you have two variables, which you attempt to keep
>> > in sync, only one of them is an atomic access and you don't have any
>> > locking or memory barriers between them, so it must be ok if they're not
>> > in-sync. In which case, why do we have two variables?
>> >
>> IMHO, two variables are being used by kgdb framework
>> The variables 'kgdb_cpu_doing_single_step'  holds the cpu on which
>> step debugging
>> is happening. With this, when during step debugging, it ensures that on next
>> step debug exception only this CPU aquires master lock and step
>> debugging is performed.
>>
>> The variable 'kgdb_single_step' is used to know if step debugging is
>> ongoing or not.
>> If so, the non-primary cpu's are not released untill the continue
>> command is received
>> So, with this the cpu's are not released and acquired for every step command
>
> Ok, but in what situation would you have both kgdb_cpu_doing_single_step == -1
> and kgdb_single_step == 1 (ignoring races between setting the two variables)?
>
> In other words, can we reduce this to a single variable?
>
More elaborately,
The kgdb_cpu_doing_single_step holds the cpu number which is used to
store the task information that is currently being step debugged in the
corresponding cpu structure. so we cannot get rid of this variable.
Also this variable tells which cpu should take the exception for step debugging
so it waits for this paricular cpu to get the chance to take exception in case
if exception has been taken by other cpu.

The kgdb_single_step variable, it only tells if cpu's that don't get
debug exception
should be released & acquired for every step debug or not

So, both variables are used for different purpose. we can make an attempt to
make to use one variable, but I think, the flexibility is lost.
Moreover, I see that these variables are declared in generic kgdb code and
different architectures use this based on their requirement.

The race will not happen because, only one cpu will be executing this
function at
any point of the time and all other cpu's will be spinning.

> Will