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Message-ID: <174670043170.406.136721948401145052.tip-bot2@tip-bot2>
Date: Thu, 08 May 2025 10:33:51 -0000
From: "tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: locking/futex] futex: Allow automatic allocation of process
wide futex hash
The following commit has been merged into the locking/futex branch of tip:
Commit-ID: 7c4f75a21f636486d2969d9b6680403ea8483539
Gitweb: https://git.kernel.org/tip/7c4f75a21f636486d2969d9b6680403ea8483539
Author: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Wed, 16 Apr 2025 18:29:13 +02:00
Committer: Peter Zijlstra <peterz@...radead.org>
CommitterDate: Sat, 03 May 2025 12:02:08 +02:00
futex: Allow automatic allocation of process wide futex hash
Allocate a private futex hash with 16 slots if a task forks its first
thread.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-14-bigeasy@linutronix.de
---
include/linux/futex.h | 6 ++++++
kernel/fork.c | 22 ++++++++++++++++++++++
kernel/futex/core.c | 11 +++++++++++
3 files changed, 39 insertions(+)
diff --git a/include/linux/futex.h b/include/linux/futex.h
index 8f1be08..1d3f755 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -80,6 +80,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4);
#ifdef CONFIG_FUTEX_PRIVATE_HASH
+int futex_hash_allocate_default(void);
void futex_hash_free(struct mm_struct *mm);
static inline void futex_mm_init(struct mm_struct *mm)
@@ -88,6 +89,7 @@ static inline void futex_mm_init(struct mm_struct *mm)
}
#else /* !CONFIG_FUTEX_PRIVATE_HASH */
+static inline int futex_hash_allocate_default(void) { return 0; }
static inline void futex_hash_free(struct mm_struct *mm) { }
static inline void futex_mm_init(struct mm_struct *mm) { }
#endif /* CONFIG_FUTEX_PRIVATE_HASH */
@@ -107,6 +109,10 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig
{
return -EINVAL;
}
+static inline int futex_hash_allocate_default(void)
+{
+ return 0;
+}
static inline void futex_hash_free(struct mm_struct *mm) { }
static inline void futex_mm_init(struct mm_struct *mm) { }
diff --git a/kernel/fork.c b/kernel/fork.c
index 831dfec..1f5d808 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2164,6 +2164,13 @@ 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;
+ return true;
+}
+
/*
* This creates a new process as a copy of the old one,
* but does not actually start it yet.
@@ -2545,6 +2552,21 @@ __latent_entropy struct task_struct *copy_process(
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 be 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
* not want user-space to be able to predict the process start-time by
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 818df74..53b3a00 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1294,6 +1294,17 @@ static int futex_hash_allocate(unsigned int hash_slots, bool custom)
return 0;
}
+int futex_hash_allocate_default(void)
+{
+ if (!current->mm)
+ return 0;
+
+ if (current->mm->futex_phash)
+ return 0;
+
+ return futex_hash_allocate(16, false);
+}
+
static int futex_hash_get_slots(void)
{
struct futex_private_hash *fph;
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