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: da91c1920242 ("linux-user: Clean up when exiting due to a signal")
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230214140829.45392-3-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/cpus-common.c b/cpus-common.c
index 793364dc0e..39f355de98 100644
--- a/cpus-common.c
+++ b/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);