Commit a59629fc authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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 <dgilbert@redhat.com>
Tested by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent e65bef69

include/qemu/rcu.h

+1 −0
Original line number Diff line number Diff line
@@ -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);

linux-user/syscall.c

+1 −0
Original line number Diff line number Diff line
@@ -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

os-posix.c

+2 −0
Original line number Diff line number Diff line
@@ -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();

    }

}

util/rcu.c

+3 −4
Original line number Diff line number Diff line
@@ -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();

}