[PATCH 0/3] gdbstub: Protect gdb_handlesig() with EXCLUSIVE_GUARD()

[PATCH 0/3] gdbstub: Protect gdb_handlesig() with EXCLUSIVE_GUARD()

[Ilya Leoshkevich]

Hi,

I ran into the issue with only one thread being stopped on a
breakpoint hit [1] again [2]. While a proper solution to this includes
kicking all threads using a reserved host signal and parking them, and
is partially in review and partially in the works, this small series
resolves most of the problems that occur during a typical debugging
session. Furthermore, it is a subset of the ultimate solution and
therefore would not have to be reverted.

Best regards,
Ilya

[1] https://gitlab.com/qemu-project/qemu/-/issues/2465
[2] https://lore.kernel.org/qemu-devel/6d1171d8debb462f468bb47ff875e0e9db253b4e.camel@linux.ibm.com/

Ilya Leoshkevich (3):
  cpu: Set current_cpu early in qemu-user
  cpu: Introduce EXCLUSIVE_GUARD()
  gdbstub: Protect gdb_handlesig() with EXCLUSIVE_GUARD()

 bsd-user/main.c       |  2 ++
 gdbstub/user.c        |  2 ++
 include/hw/core/cpu.h | 11 +++++++++++
 linux-user/main.c     |  2 ++
 linux-user/syscall.c  |  1 +
 5 files changed, 18 insertions(+)

-- 
2.47.1

[PATCH 1/3] cpu: Set current_cpu early in qemu-user

[Ilya Leoshkevich]

gdb_handlesig() uses current_cpu.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 bsd-user/main.c      | 2 ++
 linux-user/main.c    | 2 ++
 linux-user/syscall.c | 1 +
 3 files changed, 5 insertions(+)

diff --git a/bsd-user/main.c b/bsd-user/main.c
index 0a5bc578365..aa052e515c9 100644
--- a/bsd-user/main.c
+++ b/bsd-user/main.c
@@ -627,6 +627,8 @@ int main(int argc, char **argv)
 
     target_cpu_init(env, regs);
 
+    current_cpu = cpu;
+
     if (gdbstub) {
         gdbserver_start(gdbstub);
         gdb_handlesig(cpu, 0, NULL, NULL, 0);
diff --git a/linux-user/main.c b/linux-user/main.c
index b97634a32dd..b2bf0688617 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -1022,6 +1022,8 @@ int main(int argc, char **argv, char **envp)
 
     target_cpu_copy_regs(env, regs);
 
+    current_cpu = cpu;
+
     if (gdbstub) {
         if (gdbserver_start(gdbstub) < 0) {
             fprintf(stderr, "qemu: could not open gdbserver on %s\n",
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 78c7c0b34ef..a101f177632 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -6542,6 +6542,7 @@ static void *clone_func(void *arg)
     env = info->env;
     cpu = env_cpu(env);
     thread_cpu = cpu;
+    current_cpu = cpu;
     ts = get_task_state(cpu);
     info->tid = sys_gettid();
     task_settid(ts);
-- 
2.47.1

[PATCH 2/3] cpu: Introduce EXCLUSIVE_GUARD()

[Ilya Leoshkevich]

Add a macro that produces a start_exclusive() / end_exclusive() pair.
Useful to guarantee an exit from an exclusive section in large
functions.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 include/hw/core/cpu.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index c3ca0babcb3..a7d9d6e2b8c 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -1063,6 +1063,17 @@ void start_exclusive(void);
  */
 void end_exclusive(void);
 
+static inline void exclusive_unlock_guard(int *exclusive_guard G_GNUC_UNUSED)
+{
+    end_exclusive();
+}
+
+#define EXCLUSIVE_GUARD()                                      \
+    int glue(exclusive_guard, __COUNTER__)                     \
+            G_GNUC_UNUSED                                      \
+            __attribute__((cleanup(exclusive_unlock_guard))) = \
+        (start_exclusive(), 0);
+
 /**
  * qemu_init_vcpu:
  * @cpu: The vCPU to initialize.
-- 
2.47.1

[PATCH 3/3] gdbstub: Protect gdb_handlesig() with EXCLUSIVE_GUARD()

[Ilya Leoshkevich]

If multiple threads hit a breakpoint at the same time, GDB gets
confused [1]. Prevent this situation by stopping the other threads once
a thread hits a breakpoint.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=32023

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 gdbstub/user.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/gdbstub/user.c b/gdbstub/user.c
index 0b4bfa9c488..d72f8ca6106 100644
--- a/gdbstub/user.c
+++ b/gdbstub/user.c
@@ -200,6 +200,8 @@ int gdb_handlesig(CPUState *cpu, int sig, const char *reason, void *siginfo,
     char buf[256];
     int n;
 
+    EXCLUSIVE_GUARD();
+
     if (!gdbserver_state.init || gdbserver_user_state.fd < 0) {
         return sig;
     }
-- 
2.47.1