cgroups: read-write lock CLONE_THREAD forking per threadgroup

Adds functionality to read/write lock CLONE_THREAD fork()ing per-threadgroup

Add an rwsem that lives in a threadgroup's signal_struct that's taken for
reading in the fork path, under CONFIG_CGROUPS.  If another part of the
kernel later wants to use such a locking mechanism, the CONFIG_CGROUPS
ifdefs should be changed to a higher-up flag that CGROUPS and the other
system would both depend on.

This is a pre-patch for cgroup-procs-write.patch.

Signed-off-by: Ben Blum <bblum@andrew.cmu.edu>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f18300eddfcb..dc8871295a5a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -513,6 +513,7 @@ struct thread_group_cputimer {
         spinlock_t lock;
 };
 
+#include <linux/rwsem.h>
 struct autogroup;
 
 /*
@@ -632,6 +633,16 @@ struct signal_struct {
         unsigned audit_tty;
         struct tty_audit_buf *tty_audit_buf;
 #endif
+#ifdef CONFIG_CGROUPS
+        /*
+         * The threadgroup_fork_lock prevents threads from forking with
+         * CLONE_THREAD while held for writing. Use this for fork-sensitive
+         * threadgroup-wide operations. It's taken for reading in fork.c in
+         * copy_process().
+         * Currently only needed write-side by cgroups.
+         */
+        struct rw_semaphore threadgroup_fork_lock;
+#endif
 
         int oom_adj;            /* OOM kill score adjustment (bit shift) */
         int oom_score_adj;      /* OOM kill score adjustment */
@@ -2323,6 +2334,31 @@ static inline void unlock_task_sighand(struct task_struct *tsk,
         spin_unlock_irqrestore(&tsk->sighand->siglock, *flags);
 }
 
+/* See the declaration of threadgroup_fork_lock in signal_struct. */
+#ifdef CONFIG_CGROUPS
+static inline void threadgroup_fork_read_lock(struct task_struct *tsk)
+{
+        down_read(&tsk->signal->threadgroup_fork_lock);
+}
+static inline void threadgroup_fork_read_unlock(struct task_struct *tsk)
+{
+        up_read(&tsk->signal->threadgroup_fork_lock);
+}
+static inline void threadgroup_fork_write_lock(struct task_struct *tsk)
+{
+        down_write(&tsk->signal->threadgroup_fork_lock);
+}
+static inline void threadgroup_fork_write_unlock(struct task_struct *tsk)
+{
+        up_write(&tsk->signal->threadgroup_fork_lock);
+}
+#else
+static inline void threadgroup_fork_read_lock(struct task_struct *tsk) {}
+static inline void threadgroup_fork_read_unlock(struct task_struct *tsk) {}
+static inline void threadgroup_fork_write_lock(struct task_struct *tsk) {}
+static inline void threadgroup_fork_write_unlock(struct task_struct *tsk) {}
+#endif
+
 #ifndef __HAVE_THREAD_FUNCTIONS
 
 #define task_thread_info(task)  ((struct thread_info *)(task)->stack)