gdb多核调试问题

1 问题发现

在用gdb调试linux内核的网卡驱动时，在多次next之后，会出现下面结果：
执行某条语句后，gdb提示
[New Thread 4]
[New Thread 3]

contiuing ... ...
导致调试无法继续。

2 问题分析

首先想到的是产生了多线程的切换，因为调试环境是在多核机器上进行的，所以猜测其原因可能是当前核上的处于running的调试线程由于某种原因挂起了，因此调试过程切换到了其他核上，但是由于当前核上的调试资源没有被释放，因此，其他某个处于running状态的核得不到调试资源，使整个gdb挂起。今天有个师弟给我发了下面这篇文章，基本证实了这个想法。

3 kgdb多核patch

This is the updated kgdb SMP patch, which is tested on blackfin bf561 dual core
system. A new bug is fixed, which causes debugging halt in SMP kernel if a
breakpoint is hit and cpu is switched in gdb before continue running kernel.

   In blackfin SMP architecture, different core has its own L1 SRAM and MMR
   memory, which code running on the other core can't access. In current kgdb
   impelemntation, cpus are represented by thread with minus prefix.

If user run thread command in gdb to switch to the thread of the other cpu,
kgdb should:
1. send IPI signal to master cpu
2. release the specific passive cpu waiting in IPI handler
3. exit kgdb exception loop on master cpu and trap into kgdb wait in IPI handler
4. trap the released passive cpu into kgdb exception in IPI handler

Signed-off-by: Sonic Zhang <[e-mail]>
---
 include/linux/kgdb.h
 kernel/kgdb.c
Andrew Morton - Feb. 20, 2009, 12:55 a.m.
On Wed, 18 Feb 2009 17:37:54 +0800
sonic zhang <[e-mail]> wrote:

> This is the updated kgdb SMP patch, which is tested on blackfin bf561 dual core
> system. A new bug is fixed, which causes debugging halt in SMP kernel if a
> breakpoint is hit and cpu is switched in gdb before continue running kernel.
>
> In blackfin SMP architecture, different core has its own L1 SRAM and MMR
> memory, which code running on the other core can't access. In current kgdb
> impelemntation, cpus are represented by thread with minus prefix.
>
> If user run thread command in gdb to switch to the thread of the other cpu,
> kgdb should:
> 1. send IPI signal to master cpu
> 2. release the specific passive cpu waiting in IPI handler
> 3. exit kgdb exception loop on master cpu and trap into kgdb wait in IPI handler
> 4. trap the released passive cpu into kgdb exception in IPI handler
>

x86_64 allmodconfig:

kernel/kgdb.c: In function 'kgdb_wait':
kernel/kgdb.c:589: error: 'KGDB_THR_PROC_SWAP' undeclared (first use in this function)
kernel/kgdb.c:589: error: (Each undeclared identifier is reported only once
kernel/kgdb.c:589: error: for each function it appears in.)
kernel/kgdb.c: In function 'gdb_cmd_query':
kernel/kgdb.c:1087: error: 'KGDB_THR_PROC_SWAP' undeclared (first use in this function)
kernel/kgdb.c: In function 'gdb_cmd_task':
kernel/kgdb.c:1113: error: 'KGDB_THR_PROC_SWAP' undeclared (first use in this function)
kernel/kgdb.c: In function 'gdb_cmd_thread':
kernel/kgdb.c:1163: error: 'KGDB_THR_PROC_SWAP' undeclared (first use in this function)
kernel/kgdb.c: In function 'gdb_serial_stub':
kernel/kgdb.c:1275: error: 'KGDB_THR_PROC_SWAP' undeclared (first use in this function)
kernel/kgdb.c: In function 'kgdb_handle_exception':
kernel/kgdb.c:1575: error: 'KGDB_THR_PROC_SWAP' undeclared (first use in this function)
kernel/kgdb.c: In function 'kgdb_nmicallback':
kernel/kgdb.c:1606: warning: suggest parentheses around && within ||
kernel/kgdb.c:1606: warning: suggest parentheses around && within ||
kernel/kgdb.c:1606: warning: suggest parentheses around && within ||
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Patch
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index 6adcc29..664e396 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -203,6 +204,19 @@ kgdb_arch_handle_exception(int vector, int signo, int err_code,
  */
 extern void kgdb_roundup_cpus(unsigned long flags);

+/**
+ *    kgdb_roundup_cpu - Get spcific CPU into a holding pattern
+ *    @cpu: Specific cpu id
+ *    @flags: Current IRQ state
+ *
+ *    On SMP systems, we need to switch cpu from current active one to
+ *    the other passive one. This get current active CPU into a known state
+ *    in kgdb_wait().
+ *
+ *    On non-SMP systems, this is not called.
+ */
+extern void kgdb_roundup_cpu(int cpu, unsigned long flags);
+
 /* Optional functions. */
 extern int kgdb_validate_break_address(unsigned long addr);
 extern int kgdb_arch_set_breakpoint(unsigned long addr, char *saved_instr);
diff --git a/kernel/kgdb.c b/kernel/kgdb.c
index e4dcfb2..277986d 100644
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -565,6 +565,7 @@
 {
     unsigned long flags;
     int cpu;
+    struct task_struct *thread;

     local_irq_save(flags);
     cpu = raw_smp_processor_id();
@@ -577,10 +578,26 @@
     smp_wmb();
     atomic_set(&cpu_in_kgdb[cpu], 1);

+    kgdb_disable_hw_debug(regs);
+
     /* Wait till primary CPU is done with debugging */
     while (atomic_read(&passive_cpu_wait[cpu]))
         cpu_relax();

+    /* Trap into kgdb as the active CPU if gdb asks to switch. */
+    thread = getthread(regs, -raw_smp_processor_id() - 2);
+    if ((arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP) &&
+        kgdb_contthread && kgdb_contthread == current) {
+        kgdb_breakpoint();
+
+        kgdb_info[cpu].debuggerinfo = NULL;
+        kgdb_info[cpu].task = NULL;
+
+        clocksource_touch_watchdog();
+        local_irq_restore(flags);
+        return;
+    }
+
     kgdb_info[cpu].debuggerinfo = NULL;
     kgdb_info[cpu].task = NULL;

@@ -1066,13 +1083,16 @@
             sprintf(tmpstr, "shadowCPU%d",
                     (int)(-ks->threadid - 2));
             kgdb_mem2hex(tmpstr, remcom_out_buffer, strlen(tmpstr));
+
+            if (arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP)
+                ks->thr_query = 1;
         }
         break;
     }
 }

 /* Handle the 'H' task query packets */
-static void gdb_cmd_task(struct kgdb_state *ks)
+static int gdb_cmd_task(struct kgdb_state *ks)
 {
     struct task_struct *thread;
     char *ptr;
@@ -1089,6 +1109,15 @@
         kgdb_usethread = thread;
         ks->kgdb_usethreadid = ks->threadid;
         strcpy(remcom_out_buffer, "OK");
+#ifdef CONFIG_SMP
+        if ((arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP) &&
+            !ks->thr_query && ks->kgdb_usethreadid < -1 &&
+            - ks->kgdb_usethreadid - 2 != raw_smp_processor_id()) {
+            kgdb_roundup_cpu(raw_smp_processor_id(), 0);
+            kgdb_contthread = kgdb_usethread;
+            return 1;
+        }
+#endif
         break;
     case 'c':
         ptr = &remcom_in_buffer[2];
@@ -1102,10 +1131,27 @@
                 break;
             }
             kgdb_contthread = thread;
+#ifdef CONFIG_SMP
+            if ((arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP) &&
+                thread != current) {
+                int cpu;
+                for_each_online_cpu(cpu) {
+                    if (thread == kgdb_info[cpu].task) {
+                        kgdb_roundup_cpu(
+                            raw_smp_processor_id(),
+                            0);
+                        ks->kgdb_usethreadid = -cpu-2;
+                        return 1;
+                    }
+                }
+            }
+#endif
         }
         strcpy(remcom_out_buffer, "OK");
         break;
     }
+
+    return 0;
 }

 /* Handle the 'T' thread query packets */
@@ -1114,6 +1160,9 @@
     char *ptr = &remcom_in_buffer[1];
     struct task_struct *thread;

+    if (arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP)
+        ks->thr_query = 0;
+
     kgdb_hex2long(&ptr, &ks->threadid);
     thread = getthread(ks->linux_regs, ks->threadid);
     if (thread)
@@ -1223,7 +1272,12 @@
     /* Clear the out buffer. */
     memset(remcom_out_buffer, 0, sizeof(remcom_out_buffer));

-    if (kgdb_connected) {
+    if ((arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP) && kgdb_contthread) {
+        remcom_out_buffer[0] = 'O';
+        remcom_out_buffer[1] = 'K';
+        remcom_out_buffer[2] = 0;
+        put_packet(remcom_out_buffer);
+    } else if (kgdb_connected) {
         unsigned char thref[8];
         char *ptr;

@@ -1238,6 +1292,7 @@
         put_packet(remcom_out_buffer);
     }

+    kgdb_contthread = current;
     kgdb_usethread = kgdb_info[ks->cpu].task;
     ks->kgdb_usethreadid = shadow_pid(kgdb_info[ks->cpu].task->pid);
     ks->pass_exception = 0;
@@ -1284,7 +1339,8 @@
             gdb_cmd_query(ks);
             break;
         case 'H': /* task related */
-            gdb_cmd_task(ks);
+            if (gdb_cmd_task(ks))
+                goto kgdb_exit;
             break;
         case 'T': /* Query thread status */
             gdb_cmd_thread(ks);
@@ -1324,6 +1380,7 @@
             if (error >= 0 || remcom_in_buffer[0] == 'D' ||
              remcom_in_buffer[0] == 'k') {
                 error = 0;
+                kgdb_contthread = NULL;
                 goto kgdb_exit;
             }

@@ -1464,11 +1521,15 @@
      * Get the passive CPU lock which will hold all the non-primary
      * CPU in a spin state while the debugger is active
      */
-    if (!kgdb_single_step) {
+    if (!kgdb_single_step && !kgdb_contthread) {
         for (i = 0; i < NR_CPUS; i++)
             atomic_set(&passive_cpu_wait[i], 1);
     }

+    if (kgdb_contthread)
+        atomic_set(&passive_cpu_wait[raw_smp_processor_id()], 1);
+
+
     /*
      * spin_lock code is good enough as a barrier so we don't
      * need one here:
@@ -1477,7 +1538,7 @@

 #ifdef CONFIG_SMP
     /* Signal the other CPUs to enter kgdb_wait() */
-    if ((!kgdb_single_step) && kgdb_do_roundup)
+    if ((!kgdb_single_step && !kgdb_contthread) && kgdb_do_roundup)
         kgdb_roundup_cpus(flags);
 #endif

@@ -1496,7 +1557,6 @@
     kgdb_post_primary_code(ks->linux_regs, ks->ex_vector, ks->err_code);
     kgdb_deactivate_sw_breakpoints();
     kgdb_single_step = 0;
-    kgdb_contthread = current;
     exception_level = 0;

     /* Talk to debugger with gdbserial protocol */
@@ -1510,7 +1570,14 @@
     kgdb_info[ks->cpu].task = NULL;
     atomic_set(&cpu_in_kgdb[ks->cpu], 0);

-    if (!kgdb_single_step) {
+#ifdef CONFIG_SMP
+    i = -(ks->kgdb_usethreadid + 2);
+    if ((arch_kgdb_ops.flags & KGDB_THR_PROC_SWAP) &&
+        kgdb_contthread && i != cpu) {
+        atomic_set(&passive_cpu_wait[i], 0);
+    }
+#endif
+    if (!kgdb_single_step && !kgdb_contthread) {
         for (i = NR_CPUS-1; i >= 0; i--)
             atomic_set(&passive_cpu_wait[i], 0);