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

On Mon, Nov 11, 2013 at 11:09:46AM +0000, Vijay Kilari wrote:

On Fri, Nov 8, 2013 at 7:48 PM, Will Deacon [off-list ref] wrote:

quoted

On Tue, Nov 05, 2013 at 08:45:45AM +0000, vijay.kilari at gmail.com wrote:

quoted

@@ -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?

Will