include/vdso/futex.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _VDSO_FUTEX_H
#define _VDSO_FUTEX_H

#include <uapi/linux/types.h>

/**
 * __vdso_futex_robust_list64_try_unlock - Try to unlock an uncontended robust futex
 *					   with a 64-bit pending op pointer
 * @lock:	Pointer to the futex lock object
 * @tid:	The TID of the calling task
 * @pop:	Pointer to the task's robust_list_head::list_pending_op
 *
 * Return: The content of *@lock. On success this is the same as @tid.
 *
 * The function implements:
 *	if (atomic_try_cmpxchg(lock, &tid, 0))
 *		*op = NULL;
 *	return tid;
 *
 * There is a race between a successful unlock and clearing the pending op
 * pointer in the robust list head. If the calling task is interrupted in the
 * race window and has to handle a (fatal) signal on return to user space then
 * the kernel handles the clearing of @pending_op before attempting to deliver
 * the signal. That ensures that a task cannot exit with a potentially invalid
 * pending op pointer.
 *
 * User space uses it in the following way:
 *
 * if (__vdso_futex_robust_list64_try_unlock(lock, tid, &pending_op) != tid)
 *	err = sys_futex($OP | FUTEX_ROBUST_UNLOCK,....);
 *
 * If the unlock attempt fails due to the FUTEX_WAITERS bit set in the lock,
 * then the syscall does the unlock, clears the pending op pointer and wakes the
 * requested number of waiters.
 */
__u32 __vdso_futex_robust_list64_try_unlock(__u32 *lock, __u32 tid, __u64 *pop);

/**
 * __vdso_futex_robust_list32_try_unlock - Try to unlock an uncontended robust futex
 *					   with a 32-bit pending op pointer
 * @lock:	Pointer to the futex lock object
 * @tid:	The TID of the calling task
 * @pop:	Pointer to the task's robust_list_head::list_pending_op
 *
 * Return: The content of *@lock. On success this is the same as @tid.
 *
 * Same as __vdso_futex_robust_list64_try_unlock() just with a 32-bit @pop pointer.
 */
__u32 __vdso_futex_robust_list32_try_unlock(__u32 *lock, __u32 tid, __u32 *pop);

#endif