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>

1 files changed, 8 insertions, 19 deletions

diff --git a/kernel/kgdb.c b/kernel/kgdb.c
index 85b7e5b934a7..4d1b3c232377 100644
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -561,18 +561,6 @@ static void kgdb_wait(struct pt_regs *regs)
         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 @@ acquirelock:
                         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 @@ int kgdb_nmicallback(int cpu, void *regs)
 {
 #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;
         }