[PATCH v4 03/11] futex: Allow automatic allocation of process wide futex hash.

Sebastian Andrzej Siewior
2024-12-03T16:43:39Z

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

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/fork.c | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index cda8886f3a1d7..6267d600af991 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2130,6 +2130,17 @@ 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;
+ if (current->mm->futex_hash_bucket)
+ return false;
+ return true;
+}
+
/*
* This creates a new process as a copy of the old one,
* but does not actually start it yet.
@@ -2507,6 +2518,21 @@ __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