kgdb: fix SMP NMI kgdb_handle_exception exit race
Fix the problem of protecting the kgdb handle_exception exit
which had an NMI race condition, while trying to restore
normal system operation.
There was a small window after the master processor sets cpu_in_debug
to zero but before it has set kgdb_active to zero where a
non-master processor in an SMP system could receive an NMI and
re-enter the kgdb_wait() loop.
As long as the master processor sets the cpu_in_debug before sending
the cpu roundup the cpu_in_debug variable can also be used to guard
against the race condition.
The kgdb_wait() function no longer needs to check
kgdb_active because it is done in the arch specific code
and handled along with the nmi traps at the low level.
This also allows kgdb_wait() to exit correctly if it was
entered for some unknown reason due to a spurious NMI that
could not be handled by the arch specific code.
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/kgdb.c b/kernel/kgdb.c
index 85b7e5b..4d1b3c2 100644
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -561,18 +561,6 @@
smp_wmb();
atomic_set(&cpu_in_kgdb[cpu], 1);
- /*
- * The primary CPU must be active to enter here, but this is
- * guard in case the primary CPU had not been selected if
- * this was an entry via nmi.
- */
- while (atomic_read(&kgdb_active) == -1)
- cpu_relax();
-
- /* Wait till primary CPU goes completely into the debugger. */
- while (!atomic_read(&cpu_in_kgdb[atomic_read(&kgdb_active)]))
- cpu_relax();
-
/* Wait till primary CPU is done with debugging */
while (atomic_read(&passive_cpu_wait[cpu]))
cpu_relax();
@@ -1447,18 +1435,18 @@
atomic_set(&passive_cpu_wait[i], 1);
}
-#ifdef CONFIG_SMP
- /* Signal the other CPUs to enter kgdb_wait() */
- if ((!kgdb_single_step || !kgdb_contthread) && kgdb_do_roundup)
- kgdb_roundup_cpus(flags);
-#endif
-
/*
* spin_lock code is good enough as a barrier so we don't
* need one here:
*/
atomic_set(&cpu_in_kgdb[ks->cpu], 1);
+#ifdef CONFIG_SMP
+ /* Signal the other CPUs to enter kgdb_wait() */
+ if ((!kgdb_single_step || !kgdb_contthread) && kgdb_do_roundup)
+ kgdb_roundup_cpus(flags);
+#endif
+
/*
* Wait for the other CPUs to be notified and be waiting for us:
*/
@@ -1514,7 +1502,8 @@
{
#ifdef CONFIG_SMP
if (!atomic_read(&cpu_in_kgdb[cpu]) &&
- atomic_read(&kgdb_active) != cpu) {
+ atomic_read(&kgdb_active) != cpu &&
+ atomic_read(&cpu_in_kgdb[atomic_read(&kgdb_active)])) {
kgdb_wait((struct pt_regs *)regs);
return 0;
}