kgdb: eliminate kgdb_wait(), all cpus enter the same way

This is a kgdb architectural change to have all the cpus (master or
slave) enter the same function.

A cpu that hits an exception (wants to be the master cpu) will call
kgdb_handle_exception() from the trap handler and then invoke a
kgdb_roundup_cpu() to synchronize the other cpus and bring them into
the kgdb_handle_exception() as well.

A slave cpu will enter kgdb_handle_exception() from the
kgdb_nmicallback() and set the exception state to note that the
processor is a slave.

Previously the salve cpu would have called kgdb_wait().  This change
allows the debug core to change cpus without resuming the system in
order to inspect arch specific cpu information.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

1 files changed, 82 insertions, 83 deletions

diff --git a/kernel/kgdb.c b/kernel/kgdb.c
index 42fd128127a6..6882c047452d 100644
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -69,9 +69,16 @@ struct kgdb_state {
         struct pt_regs          *linux_regs;
 };
 
+/* Exception state values */
+#define DCPU_WANT_MASTER 0x1 /* Waiting to become a master kgdb cpu */
+#define DCPU_NEXT_MASTER 0x2 /* Transition from one master cpu to another */
+#define DCPU_IS_SLAVE    0x4 /* Slave cpu enter exception */
+#define DCPU_SSTEP       0x8 /* CPU is single stepping */
+
 static struct debuggerinfo_struct {
         void                    *debuggerinfo;
         struct task_struct      *task;
+        int                     exception_state;
 } kgdb_info[NR_CPUS];
 
 /**
@@ -558,49 +565,6 @@ static struct task_struct *getthread(struct pt_regs *regs, int tid)
 }
 
 /*
- * CPU debug state control:
- */
-
-#ifdef CONFIG_SMP
-static void kgdb_wait(struct pt_regs *regs)
-{
-        unsigned long flags;
-        int cpu;
-
-        local_irq_save(flags);
-        cpu = raw_smp_processor_id();
-        kgdb_info[cpu].debuggerinfo = regs;
-        kgdb_info[cpu].task = current;
-        /*
-         * 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);
-
-        /* 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]))
-                cpu_relax();
-
-        kgdb_info[cpu].debuggerinfo = NULL;
-        kgdb_info[cpu].task = NULL;
-
-        /* fix up hardware debug registers on local cpu */
-        if (arch_kgdb_ops.correct_hw_break)
-                arch_kgdb_ops.correct_hw_break();
-
-        /* Signal the primary CPU that we are done: */
-        atomic_set(&cpu_in_kgdb[cpu], 0);
-        touch_softlockup_watchdog_sync();
-        clocksource_touch_watchdog();
-        local_irq_restore(flags);
-}
-#endif
-
-/*
  * Some architectures need cache flushes when we set/clear a
  * breakpoint:
  */
@@ -1395,34 +1359,12 @@ static int kgdb_reenter_check(struct kgdb_state *ks)
         return 1;
 }
 
-/*
- * kgdb_handle_exception() - main entry point from a kernel exception
- *
- * Locking hierarchy:
- *      interface locks, if any (begin_session)
- *      kgdb lock (kgdb_active)
- */
-int
-kgdb_handle_exception(int evector, int signo, int ecode, struct pt_regs *regs)
+static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs)
 {
-        struct kgdb_state kgdb_var;
-        struct kgdb_state *ks = &kgdb_var;
         unsigned long flags;
         int sstep_tries = 100;
         int error = 0;
         int i, cpu;
-
-        ks->cpu                 = raw_smp_processor_id();
-        ks->ex_vector           = evector;
-        ks->signo               = signo;
-        ks->ex_vector           = evector;
-        ks->err_code            = ecode;
-        ks->kgdb_usethreadid    = 0;
-        ks->linux_regs          = regs;
-
-        if (kgdb_reenter_check(ks))
-                return 0; /* Ouch, double exception ! */
-
 acquirelock:
         /*
          * Interrupts will be restored by the 'trap return' code, except when
@@ -1430,13 +1372,42 @@ acquirelock:
          */
         local_irq_save(flags);
 
-        cpu = raw_smp_processor_id();
+        cpu = ks->cpu;
+        kgdb_info[cpu].debuggerinfo = regs;
+        kgdb_info[cpu].task = current;
+        /*
+         * 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);
 
         /*
-         * Acquire the kgdb_active lock:
+         * CPU will loop if it is a slave or request to become a kgdb
+         * master cpu and acquire the kgdb_active lock:
          */
-        while (atomic_cmpxchg(&kgdb_active, -1, cpu) != -1)
+        while (1) {
+                if (kgdb_info[cpu].exception_state & DCPU_WANT_MASTER) {
+                        if (atomic_cmpxchg(&kgdb_active, -1, cpu) == cpu)
+                                break;
+                } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
+                        if (!atomic_read(&passive_cpu_wait[cpu]))
+                                goto return_normal;
+                } else {
+return_normal:
+                        /* Return to normal operation by executing any
+                         * hw breakpoint fixup.
+                         */
+                        if (arch_kgdb_ops.correct_hw_break)
+                                arch_kgdb_ops.correct_hw_break();
+                        atomic_set(&cpu_in_kgdb[cpu], 0);
+                        touch_softlockup_watchdog_sync();
+                        clocksource_touch_watchdog();
+                        local_irq_restore(flags);
+                        return 0;
+                }
                 cpu_relax();
+        }
 
         /*
          * For single stepping, try to only enter on the processor
@@ -1470,9 +1441,6 @@ acquirelock:
         if (kgdb_io_ops->pre_exception)
                 kgdb_io_ops->pre_exception();
 
-        kgdb_info[ks->cpu].debuggerinfo = ks->linux_regs;
-        kgdb_info[ks->cpu].task = current;
-
         kgdb_disable_hw_debug(ks->linux_regs);
 
         /*
@@ -1484,12 +1452,6 @@ acquirelock:
                         atomic_set(&passive_cpu_wait[i], 1);
         }
 
-        /*
-         * 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_do_roundup)
@@ -1521,8 +1483,6 @@ acquirelock:
         if (kgdb_io_ops->post_exception)
                 kgdb_io_ops->post_exception();
 
-        kgdb_info[ks->cpu].debuggerinfo = NULL;
-        kgdb_info[ks->cpu].task = NULL;
         atomic_set(&cpu_in_kgdb[ks->cpu], 0);
 
         if (!kgdb_single_step) {
@@ -1555,13 +1515,52 @@ kgdb_restore:
         return error;
 }
 
+/*
+ * kgdb_handle_exception() - main entry point from a kernel exception
+ *
+ * Locking hierarchy:
+ *      interface locks, if any (begin_session)
+ *      kgdb lock (kgdb_active)
+ */
+int
+kgdb_handle_exception(int evector, int signo, int ecode, struct pt_regs *regs)
+{
+        struct kgdb_state kgdb_var;
+        struct kgdb_state *ks = &kgdb_var;
+        int ret;
+
+        ks->cpu                 = raw_smp_processor_id();
+        ks->ex_vector           = evector;
+        ks->signo               = signo;
+        ks->ex_vector           = evector;
+        ks->err_code            = ecode;
+        ks->kgdb_usethreadid    = 0;
+        ks->linux_regs          = regs;
+
+        if (kgdb_reenter_check(ks))
+                return 0; /* Ouch, double exception ! */
+        kgdb_info[ks->cpu].exception_state |= DCPU_WANT_MASTER;
+        ret = kgdb_cpu_enter(ks, regs);
+        kgdb_info[ks->cpu].exception_state &= ~DCPU_WANT_MASTER;
+        return ret;
+}
+
 int kgdb_nmicallback(int cpu, void *regs)
 {
 #ifdef CONFIG_SMP
+        struct kgdb_state kgdb_var;
+        struct kgdb_state *ks = &kgdb_var;
+
+        memset(ks, 0, sizeof(struct kgdb_state));
+        ks->cpu                 = cpu;
+        ks->linux_regs          = regs;
+
         if (!atomic_read(&cpu_in_kgdb[cpu]) &&
-                        atomic_read(&kgdb_active) != cpu &&
-                        atomic_read(&cpu_in_kgdb[atomic_read(&kgdb_active)])) {
-                kgdb_wait((struct pt_regs *)regs);
+            atomic_read(&kgdb_active) != -1 &&
+            atomic_read(&kgdb_active) != cpu) {
+                kgdb_info[cpu].exception_state |= DCPU_IS_SLAVE;
+                kgdb_cpu_enter(ks, regs);
+                kgdb_info[cpu].exception_state &= ~DCPU_IS_SLAVE;
                 return 0;
         }
 #endif