[lxc-devel] [PATCH] Use pthread_atfork() to unlock mutexes after fork()
Serge Hallyn
serge.hallyn at ubuntu.com
Wed Jan 1 19:52:33 UTC 2014
Quoting Andrey Mazo (mazo at telum.ru):
> Signed-off-by: Andrey Mazo <mazo at telum.ru>
> ---
> src/lxc/Makefile.am | 3 ++-
> src/lxc/attach.c | 2 --
> src/lxc/bdev.c | 13 -------------
> src/lxc/lxccontainer.c | 4 ----
> src/lxc/lxclock.c | 23 ++++++++++++++++++++++-
> src/lxc/monitor.c | 1 -
> 6 files changed, 24 insertions(+), 22 deletions(-)
>
> diff --git a/src/lxc/Makefile.am b/src/lxc/Makefile.am
> index 74b38e2..f18d6e1 100644
> --- 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 6749617..252e172 100644
> --- 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 b737eff..ec0d0ee 100644
> --- a/src/lxc/bdev.c
> +++ b/src/lxc/bdev.c
> @@ -72,7 +72,6 @@ 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)
> @@ -197,7 +196,6 @@ 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);
> @@ -282,7 +280,6 @@ 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);
>
> @@ -574,7 +571,6 @@ 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);
> @@ -598,7 +594,6 @@ 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);
> }
> @@ -609,7 +604,6 @@ 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);
> }
> @@ -620,7 +614,6 @@ 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);
> }
> @@ -671,7 +664,6 @@ 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;
> @@ -716,7 +708,6 @@ 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)
> @@ -861,7 +852,6 @@ 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)
> @@ -921,7 +911,6 @@ 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)
> @@ -1055,7 +1044,6 @@ 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);
> }
> @@ -2091,7 +2079,6 @@ 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 e1d004f..cf71f28 100644
> --- a/src/lxc/lxccontainer.c
> +++ b/src/lxc/lxccontainer.c
> @@ -599,7 +599,6 @@ 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) {
> @@ -837,7 +836,6 @@ 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);
> @@ -1234,7 +1232,6 @@ 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);
> @@ -2326,7 +2323,6 @@ 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 28691b0..36c4bd3 100644
> --- a/src/lxc/lxclock.c
> +++ b/src/lxc/lxclock.c
> @@ -87,7 +87,7 @@ 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);
> }
> @@ -315,6 +315,21 @@ 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.
Thanks for pointing this out. I have to say I don't like that we have
to lock this... though it should be safe with our current usage (and
I see Caglar has verified it :)
An alternative would be to have a unlock_if_locked() for each lock,
and do pthread_atfork(NULL, NULL, process_unlock_if_locked()).
There would be no risk of a race since the child is single-threaded.
But this is ok for now.
Acked-by: Serge E. Hallyn <serge.hallyn at ubuntu.com>
(Note I'm acking, but not applying until tomorrow)
> + * 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)
> {
> @@ -326,6 +341,12 @@ 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 7e0a713..0b6a065 100644
> --- a/src/lxc/monitor.c
> +++ b/src/lxc/monitor.c
> @@ -300,7 +300,6 @@ 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);
> --
> 1.8.4.5
>
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