cpus: Make {start,end}_exclusive() recursive

Currently dying to one of the core_dump_signal()s deadlocks, because
dump_core_and_abort() calls start_exclusive() two times: first via
stop_all_tasks(), and then via preexit_cleanup() ->
qemu_plugin_user_exit().

There are a number of ways to solve this: resume after dumping core;
check cpu_in_exclusive_context() in qemu_plugin_user_exit(); or make
{start,end}_exclusive() recursive. Pick the last option, since it's
the most straightforward one.

Fixes: da91c19 ("linux-user: Clean up when exiting due to a signal")
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Ilya Leoshkevich <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>

Author: iii-i

cpus-common.c

@@ -192,6 +192,11 @@ void start_exclusive(void)
     CPUState *other_cpu;
     int running_cpus;
 
+    if (current_cpu->exclusive_context_count) {
+        current_cpu->exclusive_context_count++;
+        return;
+    }
+
     qemu_mutex_lock(&qemu_cpu_list_lock);
     exclusive_idle();
 
@@ -219,13 +224,16 @@ void start_exclusive(void)
      */
     qemu_mutex_unlock(&qemu_cpu_list_lock);
 
-    current_cpu->in_exclusive_context = true;
+    current_cpu->exclusive_context_count = 1;
 }
 
 /* Finish an exclusive operation.  */
 void end_exclusive(void)
 {
-    current_cpu->in_exclusive_context = false;
+    current_cpu->exclusive_context_count--;
+    if (current_cpu->exclusive_context_count) {
+        return;
+    }
 
     qemu_mutex_lock(&qemu_cpu_list_lock);
     qatomic_set(&pending_cpus, 0);

include/hw/core/cpu.h

@@ -349,7 +349,7 @@ struct CPUState {
     bool unplug;
     bool crash_occurred;
     bool exit_request;
-    bool in_exclusive_context;
+    int exclusive_context_count;
     uint32_t cflags_next_tb;
     /* updates protected by BQL */
     uint32_t interrupt_request;
@@ -758,7 +758,7 @@ void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data
  */
 static inline bool cpu_in_exclusive_context(const CPUState *cpu)
 {
-    return cpu->in_exclusive_context;
+    return cpu->exclusive_context_count;
 }
 
 /**