rcu: do not create thread in pthread_atfork callback

If QEMU forks after the CPU threads have been created, qemu_mutex_lock_iothread will not be able to do qemu_cpu_kick_thread. There is no solution other than assuming that forks after the CPU threads have been created will end up in an exec. Forks before the CPU threads have been created (such as -daemonize) have to call rcu_after_fork manually. Notably, the oxygen theme for GTK+ forks and shows a "No such process" error without this patch. This patch can be reverted once the iothread loses the "kick the TCG thread" magic. User-mode emulation does not use the iothread, so it can also call rcu_after_fork. Reported by: Dr. David Alan Gilbert <[email protected]> Tested by: Dr. David Alan Gilbert <[email protected]> Signed-off-by: Paolo Bonzini <[email protected]>
jingzhao84 · Apr 1, 2015 · a59629f · a59629f
1 parent e65bef6
commit a59629f
Show file tree

Hide file tree

Showing 4 changed files with 7 additions and 4 deletions.
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
@@ -117,6 +117,7 @@ extern void synchronize_rcu(void);
  */
 extern void rcu_register_thread(void);
 extern void rcu_unregister_thread(void);
+extern void rcu_after_fork(void);
 
 struct rcu_head;
 typedef void RCUCBFunc(struct rcu_head *head);

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
@@ -4572,6 +4572,7 @@ static int do_fork(CPUArchState *env, unsigned int flags, abi_ulong newsp,
         ret = fork();
         if (ret == 0) {
             /* Child Process.  */
+            rcu_after_fork();
             cpu_clone_regs(env, newsp);
             fork_end(1);
             /* There is a race condition here.  The parent process could

diff --git a/os-posix.c b/os-posix.c
@@ -39,6 +39,7 @@
 #include "sysemu/sysemu.h"
 #include "net/slirp.h"
 #include "qemu-options.h"
+#include "qemu/rcu.h"
 
 #ifdef CONFIG_LINUX
 #include <sys/prctl.h>
@@ -247,6 +248,7 @@ void os_daemonize(void)
         signal(SIGTSTP, SIG_IGN);
         signal(SIGTTOU, SIG_IGN);
         signal(SIGTTIN, SIG_IGN);
+        rcu_after_fork();
     }
 }
 

diff --git a/util/rcu.c b/util/rcu.c
@@ -311,19 +311,18 @@ static void rcu_init_unlock(void)
 {
     qemu_mutex_unlock(&rcu_gp_lock);
 }
+#endif
 
-static void rcu_init_child(void)
+void rcu_after_fork(void)
 {
-    qemu_mutex_unlock(&rcu_gp_lock);
     memset(&registry, 0, sizeof(registry));
     rcu_init_complete();
 }
-#endif
 
 static void __attribute__((__constructor__)) rcu_init(void)
 {
 #ifdef CONFIG_POSIX
-    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
+    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_unlock);
 #endif
     rcu_init_complete();
 }