[RFC PATCH-for-8.2?] accel/tcg: Implement tcg_unregister_thread()

[Philippe Mathieu-Daudé]

Unplugging vCPU triggers the following assertion in
tcg_register_thread():

 796 void tcg_register_thread(void)
 797 {
 ...
 812     /* Claim an entry in tcg_ctxs */
 813     n = qatomic_fetch_inc(&tcg_cur_ctxs);
 814     g_assert(n < tcg_max_ctxs);

Implement and use tcg_unregister_thread() so when a
vCPU is unplugged, the tcg_cur_ctxs refcount is
decremented.

Reported-by: Michal Suchánek <msucha...@suse.de>
Suggested-by: Stefan Hajnoczi <stefa...@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <phi...@linaro.org>
---
RFC: untested
Report: 
https://lore.kernel.org/qemu-devel/20231204183638.gz9...@kitsune.suse.cz/
---
 include/tcg/startup.h           |  5 +++++
 accel/tcg/tcg-accel-ops-mttcg.c |  1 +
 accel/tcg/tcg-accel-ops-rr.c    |  1 +
 tcg/tcg.c                       | 17 +++++++++++++++++
 4 files changed, 24 insertions(+)

diff --git a/include/tcg/startup.h b/include/tcg/startup.h
index f71305765c..520942a4a1 100644
--- a/include/tcg/startup.h
+++ b/include/tcg/startup.h
@@ -45,6 +45,11 @@ void tcg_init(size_t tb_size, int splitwx, unsigned 
max_cpus);
  */
 void tcg_register_thread(void);
 
+/**
+ * tcg_unregister_thread: Unregister this thread with the TCG runtime
+ */
+void tcg_unregister_thread(void);
+
 /**
  * tcg_prologue_init(): Generate the code for the TCG prologue
  *
diff --git a/accel/tcg/tcg-accel-ops-mttcg.c b/accel/tcg/tcg-accel-ops-mttcg.c
index fac80095bb..88d7427aad 100644
--- a/accel/tcg/tcg-accel-ops-mttcg.c
+++ b/accel/tcg/tcg-accel-ops-mttcg.c
@@ -120,6 +120,7 @@ static void *mttcg_cpu_thread_fn(void *arg)
 
     tcg_cpus_destroy(cpu);
     qemu_mutex_unlock_iothread();
+    tcg_unregister_thread();
     rcu_remove_force_rcu_notifier(&force_rcu.notifier);
     rcu_unregister_thread();
     return NULL;
diff --git a/accel/tcg/tcg-accel-ops-rr.c b/accel/tcg/tcg-accel-ops-rr.c
index 611932f3c3..c2af3aad21 100644
--- a/accel/tcg/tcg-accel-ops-rr.c
+++ b/accel/tcg/tcg-accel-ops-rr.c
@@ -302,6 +302,7 @@ static void *rr_cpu_thread_fn(void *arg)
         rr_deal_with_unplugged_cpus();
     }
 
+    tcg_unregister_thread();
     rcu_remove_force_rcu_notifier(&force_rcu);
     rcu_unregister_thread();
     return NULL;
diff --git a/tcg/tcg.c b/tcg/tcg.c
index d2ea22b397..5125342d70 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -781,11 +781,18 @@ static void alloc_tcg_plugin_context(TCGContext *s)
  * modes.
  */
 #ifdef CONFIG_USER_ONLY
+
 void tcg_register_thread(void)
 {
     tcg_ctx = &tcg_init_ctx;
 }
+
+void tcg_unregister_thread(void)
+{
+}
+
 #else
+
 void tcg_register_thread(void)
 {
     TCGContext *s = g_malloc(sizeof(*s));
@@ -814,6 +821,16 @@ void tcg_register_thread(void)
 
     tcg_ctx = s;
 }
+
+void tcg_unregister_thread(void)
+{
+    unsigned int n;
+
+    n = qatomic_fetch_dec(&tcg_cur_ctxs);
+    g_free(tcg_ctxs[n]);
+    qatomic_set(&tcg_ctxs[n], NULL);
+}
+
 #endif /* !CONFIG_USER_ONLY */
 
 /* pool based memory allocation */
-- 
2.41.0