Allocate a default futex hash if a task forks its first thread.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/futex.h | 12 ++++++++++++
 kernel/fork.c         | 24 ++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 943828db52234..bad377c30de5e 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -86,6 +86,13 @@ static inline void futex_mm_init(struct mm_struct *mm)
 	mm->futex_hash_bucket = NULL;
 }
 
+static inline bool futex_hash_requires_allocation(void)
+{
+	if (current->mm->futex_hash_bucket)
+		return false;
+	return true;
+}
+
 #else
 static inline void futex_init_task(struct task_struct *tsk) { }
 static inline void futex_exit_recursive(struct task_struct *tsk) { }
@@ -108,6 +115,11 @@ static inline int futex_hash_allocate_default(void)
 static inline void futex_hash_free(struct mm_struct *mm) { }
 static inline void futex_mm_init(struct mm_struct *mm) { }
 
+static inline bool futex_hash_requires_allocation(void)
+{
+	return false;
+}
+
 #endif
 
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index cda8886f3a1d7..e34bb2a107a9d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2130,6 +2130,15 @@ static void rv_task_fork(struct task_struct *p)
 #define rv_task_fork(p) do {} while (0)
 #endif
 
+static bool need_futex_hash_allocate_default(u64 clone_flags)
+{
+	if ((clone_flags & (CLONE_THREAD | CLONE_VM)) != (CLONE_THREAD | CLONE_VM))
+		return false;
+	if (!thread_group_empty(current))
+		return false;
+	return futex_hash_requires_allocation();
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -2507,6 +2516,21 @@ __latent_entropy struct task_struct *copy_process(
 	if (retval)
 		goto bad_fork_cancel_cgroup;
 
+	/*
+	 * Allocate a default futex hash for the user process once the first
+	 * thread spawns.
+	 */
+	if (need_futex_hash_allocate_default(clone_flags)) {
+		retval = futex_hash_allocate_default();
+		if (retval)
+			goto bad_fork_core_free;
+		/*
+		 * If we fail beyond this point we don't free the allocated
+		 * futex hash map. We assume that another thread will created
+		 * and makes use of it The hash map will be freed once the main
+		 * thread terminates.
+		 */
+	}
 	/*
 	 * From this point on we must avoid any synchronous user-space
 	 * communication until we take the tasklist-lock. In particular, we do
-- 
2.45.2