linux-kernel - [RFC PATCH v2 4/4] futex: Allow automatic allocation of process wide futex hash.

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Message-ID: <20241028121921.1264150-5-bigeasy@linutronix.de>
Date: Mon, 28 Oct 2024 13:13:58 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [RFC PATCH v2 4/4] futex: Allow automatic allocation of process wide futex hash.

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

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/futex.h |  6 ++++++
 kernel/fork.c         | 28 ++++++++++++++++++++++++++++
 kernel/futex/core.c   |  5 +++++
 3 files changed, 39 insertions(+)

diff --git a/include/linux/futex.h b/include/linux/futex.h
index dad50173f70c4..c0f90dda6a295 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -79,6 +79,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 	      u32 __user *uaddr2, u32 val2, u32 val3);
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3,
 		     unsigned long arg4, unsigned long arg5);
+int futex_hash_allocate_default(void);
 #else
 static inline void futex_init_task(struct task_struct *tsk) { }
 static inline void futex_exit_recursive(struct task_struct *tsk) { }
@@ -95,6 +96,11 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3,
 {
 	return -EINVAL;
 }
+static inline int futex_hash_allocate_default(void)
+{
+	return 0;
+}
+
 #endif
 
 #endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 0d2b0a5299bbc..21dccdc8a1f6c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2107,6 +2107,19 @@ 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_leader(current))
+		return false;
+	if (current->signal->nr_threads != 1)
+		return false;
+	if (current->signal->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.
@@ -2483,6 +2496,21 @@ __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
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 3ef4cbd5cfa72..8896ade418b4a 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1204,6 +1204,11 @@ static int futex_hash_allocate(unsigned long arg3, unsigned long arg4,
 	return 0;
 }
 
+int futex_hash_allocate_default(void)
+{
+	return futex_hash_allocate(0, 0, 0);
+}
+
 static int futex_hash_is_shared(unsigned long arg3, unsigned long arg4,
 				unsigned long arg5)
 {
-- 
2.45.2