diff options
Diffstat (limited to 'kernel/futex.c')
| -rw-r--r-- | kernel/futex.c | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 5efa2f978032..feb724b2554e 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -8,6 +8,10 @@ * Removed page pinning, fix privately mapped COW pages and other cleanups * (C) Copyright 2003, 2004 Jamie Lokier * + * Robust futex support started by Ingo Molnar + * (C) Copyright 2006 Red Hat Inc, All Rights Reserved + * Thanks to Thomas Gleixner for suggestions, analysis and fixes. + * * Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly * enough at me, Linus for the original (flawed) idea, Matthew * Kirkwood for proof-of-concept implementation. @@ -829,6 +833,174 @@ error: goto out; } +/* + * Support for robust futexes: the kernel cleans up held futexes at + * thread exit time. + * + * Implementation: user-space maintains a per-thread list of locks it + * is holding. Upon do_exit(), the kernel carefully walks this list, + * and marks all locks that are owned by this thread with the + * FUTEX_OWNER_DEAD bit, and wakes up a waiter (if any). The list is + * always manipulated with the lock held, so the list is private and + * per-thread. Userspace also maintains a per-thread 'list_op_pending' + * field, to allow the kernel to clean up if the thread dies after + * acquiring the lock, but just before it could have added itself to + * the list. There can only be one such pending lock. + */ + +/** + * sys_set_robust_list - set the robust-futex list head of a task + * @head: pointer to the list-head + * @len: length of the list-head, as userspace expects + */ +asmlinkage long +sys_set_robust_list(struct robust_list_head __user *head, + size_t len) +{ + /* + * The kernel knows only one size for now: + */ + if (unlikely(len != sizeof(*head))) + return -EINVAL; + + current->robust_list = head; + + return 0; +} + +/** + * sys_get_robust_list - get the robust-futex list head of a task + * @pid: pid of the process [zero for current task] + * @head_ptr: pointer to a list-head pointer, the kernel fills it in + * @len_ptr: pointer to a length field, the kernel fills in the header size + */ +asmlinkage long +sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, + size_t __user *len_ptr) +{ + struct robust_list_head *head; + unsigned long ret; + + if (!pid) + head = current->robust_list; + else { + struct task_struct *p; + + ret = -ESRCH; + read_lock(&tasklist_lock); + p = find_task_by_pid(pid); + if (!p) + goto err_unlock; + ret = -EPERM; + if ((current->euid != p->euid) && (current->euid != p->uid) && + !capable(CAP_SYS_PTRACE)) + goto err_unlock; + head = p->robust_list; + read_unlock(&tasklist_lock); + } + + if (put_user(sizeof(*head), len_ptr)) + return -EFAULT; + return put_user(head, head_ptr); + +err_unlock: + read_unlock(&tasklist_lock); + + return ret; +} + +/* + * Process a futex-list entry, check whether it's owned by the + * dying task, and do notification if so: + */ +int handle_futex_death(unsigned int *uaddr, struct task_struct *curr) +{ + unsigned int futex_val; + +repeat: + if (get_user(futex_val, uaddr)) + return -1; + + if ((futex_val & FUTEX_TID_MASK) == curr->pid) { + /* + * Ok, this dying thread is truly holding a futex + * of interest. Set the OWNER_DIED bit atomically + * via cmpxchg, and if the value had FUTEX_WAITERS + * set, wake up a waiter (if any). (We have to do a + * futex_wake() even if OWNER_DIED is already set - + * to handle the rare but possible case of recursive + * thread-death.) The rest of the cleanup is done in + * userspace. + */ + if (futex_atomic_cmpxchg_inuser(uaddr, futex_val, + futex_val | FUTEX_OWNER_DIED) != + futex_val) + goto repeat; + + if (futex_val & FUTEX_WAITERS) + futex_wake((unsigned long)uaddr, 1); + } + return 0; +} + +/* + * Walk curr->robust_list (very carefully, it's a userspace list!) + * and mark any locks found there dead, and notify any waiters. + * + * We silently return on any sign of list-walking problem. + */ +void exit_robust_list(struct task_struct *curr) +{ + struct robust_list_head __user *head = curr->robust_list; + struct robust_list __user *entry, *pending; + unsigned int limit = ROBUST_LIST_LIMIT; + unsigned long futex_offset; + + /* + * Fetch the list head (which was registered earlier, via + * sys_set_robust_list()): + */ + if (get_user(entry, &head->list.next)) + return; + /* + * Fetch the relative futex offset: + */ + if (get_user(futex_offset, &head->futex_offset)) + return; + /* + * Fetch any possibly pending lock-add first, and handle it + * if it exists: + */ + if (get_user(pending, &head->list_op_pending)) + return; + if (pending) + handle_futex_death((void *)pending + futex_offset, curr); + + while (entry != &head->list) { + /* + * A pending lock might already be on the list, so + * dont process it twice: + */ + if (entry != pending) + if (handle_futex_death((void *)entry + futex_offset, + curr)) + return; + + /* + * Fetch the next entry in the list: + */ + if (get_user(entry, &entry->next)) + return; + /* + * Avoid excessively long or circular lists: + */ + if (!--limit) + break; + + cond_resched(); + } +} + long do_futex(unsigned long uaddr, int op, int val, unsigned long timeout, unsigned long uaddr2, int val2, int val3) { |