Use pthread_atfork() to unlock mutexes after fork()

Signed-off-by: Andrey Mazo <mazo@telum.ru>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am
index 74b38e2dbbfa2ee285d3f0c5611591ccdc9465b0..f18d6e17689df779486eff1aee0420d7026669e2 100644 (file)
--- a/src/lxc/Makefile.am
+++ b/src/lxc/Makefile.am
@@ -135,9 +135,10 @@ AM_CFLAGS += -DHAVE_SECCOMP
 liblxc_so_SOURCES += seccomp.c
 endif
 
-liblxc_so_CFLAGS = -fPIC -DPIC $(AM_CFLAGS)
+liblxc_so_CFLAGS = -fPIC -DPIC $(AM_CFLAGS) -pthread
 
 liblxc_so_LDFLAGS = \
+       -pthread \
        -shared \
        -Wl,-soname,liblxc.so.$(firstword $(subst ., ,$(VERSION)))
 

diff --git a/src/lxc/attach.c b/src/lxc/attach.c
index 674961731b39f8e58ab7499769b60a2a7bc91c5b..252e172a2a97d0b73b1da1f03bdebddf5f06b365 100644 (file)
--- a/src/lxc/attach.c
+++ b/src/lxc/attach.c
@@ -497,7 +497,6 @@ char *lxc_attach_getpwshell(uid_t uid)
                        NULL
                };
 
-               process_unlock(); // we're no longer sharing
                close(pipes[0]);
 
                /* we want to capture stdout */
@@ -790,7 +789,6 @@ int lxc_attach(const char* name, const char* lxcpath, lxc_attach_exec_t exec_fun
                return -1;
        }
 
-       process_unlock(); // we're no longer sharing
        /* first subprocess begins here, we close the socket that is for the
         * initial thread
         */

diff --git a/src/lxc/bdev.c b/src/lxc/bdev.c
index 29e2ad57f77b4d9c594df2380ed3efdc7fd89e99..d6175c111771aa95ae4a128f2e04d259774d7d37 100644 (file)
--- a/src/lxc/bdev.c
+++ b/src/lxc/bdev.c
@@ -73,7 +73,6 @@ static int do_rsync(const char *src, const char *dest)
        if (pid > 0)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        l = strlen(src) + 2;
        s = malloc(l);
        if (!s)
@@ -198,7 +197,6 @@ static int do_mkfs(const char *path, const char *fstype)
        if (pid > 0)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        // If the file is not a block device, we don't want mkfs to ask
        // us about whether to proceed.
        close(0);
@@ -283,7 +281,6 @@ static int detect_fs(struct bdev *bdev, char *type, int len)
                return ret;
        }
 
-       process_unlock(); // we're no longer sharing
        if (unshare(CLONE_NEWNS) < 0)
                exit(1);
 
@@ -575,7 +572,6 @@ static int zfs_clone(const char *opath, const char *npath, const char *oname,
                if (!pid) {
                        char dev[MAXPATHLEN];
 
-                       process_unlock(); // we're no longer sharing
                        ret = snprintf(dev, MAXPATHLEN, "%s/%s", zfsroot, nname);
                        if (ret < 0  || ret >= MAXPATHLEN)
                                exit(1);
@@ -599,7 +595,6 @@ static int zfs_clone(const char *opath, const char *npath, const char *oname,
                if ((pid = fork()) < 0)
                        return -1;
                if (!pid) {
-                       process_unlock(); // we're no longer sharing
                        execlp("zfs", "zfs", "destroy", path1, NULL);
                        exit(1);
                }
@@ -610,7 +605,6 @@ static int zfs_clone(const char *opath, const char *npath, const char *oname,
                if ((pid = fork()) < 0)
                        return -1;
                if (!pid) {
-                       process_unlock(); // we're no longer sharing
                        execlp("zfs", "zfs", "snapshot", path1, NULL);
                        exit(1);
                }
@@ -621,7 +615,6 @@ static int zfs_clone(const char *opath, const char *npath, const char *oname,
                if ((pid = fork()) < 0)
                        return -1;
                if (!pid) {
-                       process_unlock(); // we're no longer sharing
                        execlp("zfs", "zfs", "clone", option, path1, path2, NULL);
                        exit(1);
                }
@@ -672,7 +665,6 @@ static int zfs_destroy(struct bdev *orig)
        if (pid)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        if (!zfs_list_entry(orig->src, output, MAXPATHLEN)) {
                ERROR("Error: zfs entry for %s not found", orig->src);
                return -1;
@@ -717,7 +709,6 @@ static int zfs_create(struct bdev *bdev, const char *dest, const char *n,
        if (pid)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        char dev[MAXPATHLEN];
        ret = snprintf(dev, MAXPATHLEN, "%s/%s", zfsroot, n);
        if (ret < 0  || ret >= MAXPATHLEN)
@@ -862,7 +853,6 @@ static int do_lvm_create(const char *path, unsigned long size, const char *thinp
        if (pid > 0)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        // lvcreate default size is in M, not bytes.
        ret = snprintf(sz, 24, "%lu", size/1000000);
        if (ret < 0 || ret >= 24)
@@ -922,7 +912,6 @@ static int lvm_snapshot(const char *orig, const char *path, unsigned long size)
        if (pid > 0)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        // lvcreate default size is in M, not bytes.
        ret = snprintf(sz, 24, "%lu", size/1000000);
        if (ret < 0 || ret >= 24)
@@ -1056,7 +1045,6 @@ static int lvm_destroy(struct bdev *orig)
        if ((pid = fork()) < 0)
                return -1;
        if (!pid) {
-               process_unlock(); // we're no longer sharing
                execlp("lvremove", "lvremove", "-f", orig->src, NULL);
                exit(1);
        }
@@ -2092,7 +2080,6 @@ struct bdev *bdev_copy(const char *src, const char *oldname, const char *cname,
                return new;
        }
 
-       process_unlock(); // we're no longer sharing
        if (unshare(CLONE_NEWNS) < 0) {
                SYSERROR("unshare CLONE_NEWNS");
                exit(1);

diff --git a/src/lxc/lxccontainer.c b/src/lxc/lxccontainer.c
index b62f7ba6eb861a180758a4453a994f63a5fb7e77..e1cbd76f5340fe78f4ce1bebb5d4bd87f4aee469 100644 (file)
--- a/src/lxc/lxccontainer.c
+++ b/src/lxc/lxccontainer.c
@@ -601,7 +601,6 @@ static bool lxcapi_start(struct lxc_container *c, int useinit, char * const argv
                if (pid != 0)
                        return wait_on_daemonized_start(c, pid);
 
-               process_unlock(); // we're no longer sharing
                /* second fork to be reparented by init */
                pid = fork();
                if (pid < 0) {
@@ -839,7 +838,6 @@ static bool create_run_template(struct lxc_container *c, char *tpath, bool quiet
                char **newargv;
                struct lxc_conf *conf = c->lxc_conf;
 
-               process_unlock(); // we're no longer sharing
                if (quiet) {
                        close(0);
                        close(1);
@@ -1236,7 +1234,6 @@ static bool lxcapi_create(struct lxc_container *c, const char *t,
        if (pid == 0) { // child
                struct bdev *bdev = NULL;
 
-               process_unlock(); // we're no longer sharing
                if (!(bdev = do_bdev_create(c, bdevtype, specs))) {
                        ERROR("Error creating backing store type %s for %s",
                                bdevtype ? bdevtype : "(none)", c->name);
@@ -2328,7 +2325,6 @@ static int clone_update_rootfs(struct lxc_container *c0,
        if (pid > 0)
                return wait_for_pid(pid);
 
-       process_unlock(); // we're no longer sharing
        bdev = bdev_init(c->lxc_conf->rootfs.path, c->lxc_conf->rootfs.mount, NULL);
        if (!bdev)
                exit(1);

diff --git a/src/lxc/lxclock.c b/src/lxc/lxclock.c
index b30f4c3421079b0305fba0de76973abde233fc4a..69d3d158af9bd73ef4391b8a4cba23e32e4054fd 100644 (file)
--- a/src/lxc/lxclock.c
+++ b/src/lxc/lxclock.c
@@ -89,7 +89,7 @@ void unlock_mutex(pthread_mutex_t *l)
        int ret;
 
        if ((ret = pthread_mutex_unlock(l)) != 0) {
-               fprintf(stderr, "pthread_mutex_lock returned:%d %s", ret, strerror(ret));
+               fprintf(stderr, "pthread_mutex_unlock returned:%d %s", ret, strerror(ret));
                dump_stacktrace();
                exit(1);
        }
@@ -317,6 +317,21 @@ void process_unlock(void)
        unlock_mutex(&thread_mutex);
 }
 
+/* One thread can do fork() while another one is holding a mutex.
+ * There is only one thread in child just after the fork(), so noone will ever release that mutex.
+ * We setup a "child" fork handler to unlock the mutex just after the fork().
+ * For several mutex types, unlocking an unlocked mutex can lead to undefined behavior.
+ * One way to deal with it is to setup "prepare" fork handler
+ * to lock the mutex before fork() and both "parent" and "child" fork handlers
+ * to unlock the mutex.
+ * This forbids doing fork() while explicitly holding the lock.
+ */
+__attribute__((constructor))
+static void process_lock_setup_atfork(void)
+{
+       pthread_atfork(process_lock, process_unlock, process_unlock);
+}
+
 /* Protects static const values inside the lxc_global_config_value funtion */
 void static_lock(void)
 {
@@ -328,6 +343,12 @@ void static_unlock(void)
        unlock_mutex(&static_mutex);
 }
 
+__attribute__((constructor))
+static void static_lock_setup_atfork(void)
+{
+       pthread_atfork(static_lock, static_unlock, static_unlock);
+}
+
 int container_mem_lock(struct lxc_container *c)
 {
        return lxclock(c->privlock, 0);

diff --git a/src/lxc/monitor.c b/src/lxc/monitor.c
index 2cc4831afd7c472b46a17460de570278475fcbcd..1fe117072b28e4e9ccf2a77182942708b5f154e3 100644 (file)
--- a/src/lxc/monitor.c
+++ b/src/lxc/monitor.c
@@ -299,7 +299,6 @@ int lxc_monitord_spawn(const char *lxcpath)
                return 0;
        }
 
-       process_unlock(); // we're no longer sharing
        if (pipe(pipefd) < 0) {
                SYSERROR("failed to create pipe");
                exit(EXIT_FAILURE);