[PATCH v3 5/5] plugins: make qemu_plugin_user_exit's locking order consistent with fork_start's

From: Emilio Cota <cota@braap.org>
To: qemu-devel@nongnu.org
Cc: "Alex Bennée" <alex.bennee@linaro.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Philippe Mathieu-Daudé" <philmd@linaro.org>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Emilio Cota" <cota@braap.org>
Subject: [PATCH v3 5/5] plugins: make qemu_plugin_user_exit's locking order consistent with fork_start's
Date: Wed, 11 Jan 2023 10:16:28 -0500	[thread overview]
Message-ID: <20230111151628.320011-6-cota@braap.org> (raw)
In-Reply-To: <20230111151628.320011-1-cota@braap.org>

To fix potential deadlocks as reported by tsan.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Emilio Cota <cota@braap.org>
---
 plugins/core.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/plugins/core.c b/plugins/core.c
index ccb770a485..728bacef95 100644
--- a/plugins/core.c
+++ b/plugins/core.c
@@ -500,10 +500,17 @@ void qemu_plugin_user_exit(void)
     enum qemu_plugin_event ev;
     CPUState *cpu;
 
-    QEMU_LOCK_GUARD(&plugin.lock);
-
+    /*
+     * Locking order: we must acquire locks in an order that is consistent
+     * with the one in fork_start(). That is:
+     * - start_exclusive(), which acquires qemu_cpu_list_lock,
+     *   must be called before acquiring plugin.lock.
+     * - tb_flush(), which acquires mmap_lock(), must be called
+     *   while plugin.lock is not held.
+     */
     start_exclusive();
 
+    qemu_rec_mutex_lock(&plugin.lock);
     /* un-register all callbacks except the final AT_EXIT one */
     for (ev = 0; ev < QEMU_PLUGIN_EV_MAX; ev++) {
         if (ev != QEMU_PLUGIN_EV_ATEXIT) {
@@ -513,13 +520,12 @@ void qemu_plugin_user_exit(void)
             }
         }
     }
-
-    tb_flush(current_cpu);
-
     CPU_FOREACH(cpu) {
         qemu_plugin_disable_mem_helpers(cpu);
     }
+    qemu_rec_mutex_unlock(&plugin.lock);
 
+    tb_flush(current_cpu);
     end_exclusive();
 
     /* now it's safe to handle the exit case */
-- 
2.34.1


