From a0c1e9073ef7428a14309cba010633a6cd6719ea Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Sat, 23 Feb 2008 15:23:57 -0800 Subject: futex: runtime enable pi and robust functionality Not all architectures implement futex_atomic_cmpxchg_inatomic(). The default implementation returns -ENOSYS, which is currently not handled inside of the futex guts. Futex PI calls and robust list exits with a held futex result in an endless loop in the futex code on architectures which have no support. Fixing up every place where futex_atomic_cmpxchg_inatomic() is called would add a fair amount of extra if/else constructs to the already complex code. It is also not possible to disable the robust feature before user space tries to register robust lists. Compile time disabling is not a good idea either, as there are already architectures with runtime detection of futex_atomic_cmpxchg_inatomic support. Detect the functionality at runtime instead by calling cmpxchg_futex_value_locked() with a NULL pointer from the futex initialization code. This is guaranteed to fail, but the call of futex_atomic_cmpxchg_inatomic() happens with pagefaults disabled. On architectures, which use the asm-generic implementation or have a runtime CPU feature detection, a -ENOSYS return value disables the PI/robust features. On architectures with a working implementation the call returns -EFAULT and the PI/robust features are enabled. The relevant syscalls return -ENOSYS and the robust list exit code is blocked, when the detection fails. Fixes http://lkml.org/lkml/2008/2/11/149 Originally reported by: Lennart Buytenhek Signed-off-by: Thomas Gleixner Acked-by: Ingo Molnar Cc: Lennert Buytenhek Cc: Riku Voipio Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/futex.c | 38 ++++++++++++++++++++++++++++++++++---- 1 file changed, 34 insertions(+), 4 deletions(-) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index c21f667c63f..06968cd7920 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -60,6 +60,8 @@ #include "rtmutex_common.h" +int __read_mostly futex_cmpxchg_enabled; + #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8) /* @@ -469,6 +471,8 @@ void exit_pi_state_list(struct task_struct *curr) struct futex_hash_bucket *hb; union futex_key key; + if (!futex_cmpxchg_enabled) + return; /* * We are a ZOMBIE and nobody can enqueue itself on * pi_state_list anymore, but we have to be careful @@ -1870,6 +1874,8 @@ asmlinkage long sys_set_robust_list(struct robust_list_head __user *head, size_t len) { + if (!futex_cmpxchg_enabled) + return -ENOSYS; /* * The kernel knows only one size for now: */ @@ -1894,6 +1900,9 @@ sys_get_robust_list(int pid, struct robust_list_head __user * __user *head_ptr, struct robust_list_head __user *head; unsigned long ret; + if (!futex_cmpxchg_enabled) + return -ENOSYS; + if (!pid) head = current->robust_list; else { @@ -1997,6 +2006,9 @@ void exit_robust_list(struct task_struct *curr) unsigned long futex_offset; int rc; + if (!futex_cmpxchg_enabled) + return; + /* * Fetch the list head (which was registered earlier, via * sys_set_robust_list()): @@ -2051,7 +2063,7 @@ void exit_robust_list(struct task_struct *curr) long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, u32 __user *uaddr2, u32 val2, u32 val3) { - int ret; + int ret = -ENOSYS; int cmd = op & FUTEX_CMD_MASK; struct rw_semaphore *fshared = NULL; @@ -2083,13 +2095,16 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3); break; case FUTEX_LOCK_PI: - ret = futex_lock_pi(uaddr, fshared, val, timeout, 0); + if (futex_cmpxchg_enabled) + ret = futex_lock_pi(uaddr, fshared, val, timeout, 0); break; case FUTEX_UNLOCK_PI: - ret = futex_unlock_pi(uaddr, fshared); + if (futex_cmpxchg_enabled) + ret = futex_unlock_pi(uaddr, fshared); break; case FUTEX_TRYLOCK_PI: - ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1); + if (futex_cmpxchg_enabled) + ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1); break; default: ret = -ENOSYS; @@ -2145,8 +2160,23 @@ static struct file_system_type futex_fs_type = { static int __init init(void) { + u32 curval; int i; + /* + * This will fail and we want it. Some arch implementations do + * runtime detection of the futex_atomic_cmpxchg_inatomic() + * functionality. We want to know that before we call in any + * of the complex code paths. Also we want to prevent + * registration of robust lists in that case. NULL is + * guaranteed to fault and we get -EFAULT on functional + * implementation, the non functional ones will return + * -ENOSYS. + */ + curval = cmpxchg_futex_value_locked(NULL, 0, 0); + if (curval == -EFAULT) + futex_cmpxchg_enabled = 1; + for (i = 0; i < ARRAY_SIZE(futex_queues); i++) { plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock); spin_lock_init(&futex_queues[i].lock); -- cgit v1.2.3-18-g5258