Commit 0771dfef authored by Ingo Molnar's avatar Ingo Molnar Committed by Linus Torvalds

[PATCH] lightweight robust futexes: core

Add the core infrastructure for robust futexes: structure definitions, the new
syscalls and the do_exit() based cleanup mechanism.
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarArjan van de Ven <arjan@infradead.org>
Acked-by: default avatarUlrich Drepper <drepper@redhat.com>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent e9056f13
#ifndef _LINUX_FUTEX_H
#define _LINUX_FUTEX_H
#include <linux/sched.h>
/* Second argument to futex syscall */
......@@ -11,10 +13,103 @@
#define FUTEX_CMP_REQUEUE 4
#define FUTEX_WAKE_OP 5
/*
* Support for robust futexes: the kernel cleans up held futexes at
* thread exit time.
*/
/*
* Per-lock list entry - embedded in user-space locks, somewhere close
* to the futex field. (Note: user-space uses a double-linked list to
* achieve O(1) list add and remove, but the kernel only needs to know
* about the forward link)
*
* NOTE: this structure is part of the syscall ABI, and must not be
* changed.
*/
struct robust_list {
struct robust_list __user *next;
};
/*
* Per-thread list head:
*
* NOTE: this structure is part of the syscall ABI, and must only be
* changed if the change is first communicated with the glibc folks.
* (When an incompatible change is done, we'll increase the structure
* size, which glibc will detect)
*/
struct robust_list_head {
/*
* The head of the list. Points back to itself if empty:
*/
struct robust_list list;
/*
* This relative offset is set by user-space, it gives the kernel
* the relative position of the futex field to examine. This way
* we keep userspace flexible, to freely shape its data-structure,
* without hardcoding any particular offset into the kernel:
*/
long futex_offset;
/*
* The death of the thread may race with userspace setting
* up a lock's links. So to handle this race, userspace first
* sets this field to the address of the to-be-taken lock,
* then does the lock acquire, and then adds itself to the
* list, and then clears this field. Hence the kernel will
* always have full knowledge of all locks that the thread
* _might_ have taken. We check the owner TID in any case,
* so only truly owned locks will be handled.
*/
struct robust_list __user *list_op_pending;
};
/*
* Are there any waiters for this robust futex:
*/
#define FUTEX_WAITERS 0x80000000
/*
* The kernel signals via this bit that a thread holding a futex
* has exited without unlocking the futex. The kernel also does
* a FUTEX_WAKE on such futexes, after setting the bit, to wake
* up any possible waiters:
*/
#define FUTEX_OWNER_DIED 0x40000000
/*
* Reserved bit:
*/
#define FUTEX_OWNER_PENDING 0x20000000
/*
* The rest of the robust-futex field is for the TID:
*/
#define FUTEX_TID_MASK 0x1fffffff
/*
* A limit of one million locks held per thread (!) ought to be enough
* for some time. This also protects against a deliberately circular
* list. Not worth introducing an rlimit for this:
*/
#define ROBUST_LIST_LIMIT 1048576
long do_futex(unsigned long uaddr, int op, int val,
unsigned long timeout, unsigned long uaddr2, int val2,
int val3);
extern int handle_futex_death(unsigned int *uaddr, struct task_struct *curr);
#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
#else
static inline void exit_robust_list(struct task_struct *curr)
{
}
#endif
#define FUTEX_OP_SET 0 /* *(int *)UADDR2 = OPARG; */
#define FUTEX_OP_ADD 1 /* *(int *)UADDR2 += OPARG; */
#define FUTEX_OP_OR 2 /* *(int *)UADDR2 |= OPARG; */
......
......@@ -35,6 +35,7 @@
#include <linux/topology.h>
#include <linux/seccomp.h>
#include <linux/rcupdate.h>
#include <linux/futex.h>
#include <linux/auxvec.h> /* For AT_VECTOR_SIZE */
......@@ -872,6 +873,8 @@ struct task_struct {
int cpuset_mems_generation;
int cpuset_mem_spread_rotor;
#endif
struct robust_list_head __user *robust_list;
atomic_t fs_excl; /* holding fs exclusive resources */
struct rcu_head rcu;
};
......
......@@ -28,7 +28,8 @@
#define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)
/*
* A maximum of 4 million PIDs should be enough for a while:
* A maximum of 4 million PIDs should be enough for a while.
* [NOTE: PID/TIDs are limited to 2^29 ~= 500+ million, see futex.h.]
*/
#define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
(sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))
......
......@@ -31,6 +31,7 @@
#include <linux/signal.h>
#include <linux/cn_proc.h>
#include <linux/mutex.h>
#include <linux/futex.h>
#include <asm/uaccess.h>
#include <asm/unistd.h>
......@@ -852,6 +853,8 @@ fastcall NORET_TYPE void do_exit(long code)
exit_itimers(tsk->signal);
acct_process(code);
}
if (unlikely(tsk->robust_list))
exit_robust_list(tsk);
exit_mm(tsk);
exit_sem(tsk);
......
......@@ -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 @@ static int futex_fd(unsigned long uaddr, int signal)
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)
{
......
......@@ -42,6 +42,10 @@ cond_syscall(sys_recvmsg);
cond_syscall(sys_socketcall);
cond_syscall(sys_futex);
cond_syscall(compat_sys_futex);
cond_syscall(sys_set_robust_list);
cond_syscall(compat_sys_set_robust_list);
cond_syscall(sys_get_robust_list);
cond_syscall(compat_sys_get_robust_list);
cond_syscall(sys_epoll_create);
cond_syscall(sys_epoll_ctl);
cond_syscall(sys_epoll_wait);
......
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