Commit 7c4f75a2 authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Peter Zijlstra
Browse files

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: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250416162921.513656-14-bigeasy@linutronix.de
parent 80367ad0

include/linux/futex.h

+6 −0
Original line number Diff line number Diff line
@@ -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) { }

kernel/fork.c

+22 −0
Original line number Diff line number Diff line
@@ -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.
@@ -2544,6 +2551,21 @@ __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 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

kernel/futex/core.c

+11 −0
Original line number Diff line number Diff line
@@ -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;