Re: [patch 33/76] futex: runtime enable pi and robust functionality

[Benjamin Herrenschmidt]

On Fri, 2008-03-21 at 15:43 -0700, Chris Wright wrote:
> plain text document attachment
> (futex-runtime-enable-pi-and-robust-functionality.patch)
> -stable review patch.  If anyone has any objections, please let us know.
> ---------------------
> 
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> Not all architectures implement futex_atomic_cmpxchg_inatomic().  The default
> implementation returns -ENOSYS, which is currently not handled inside of the
> futex guts.

I think that breaks some embedded PowerPC platforms. It's also very
fishy... So early, we haven't activated an mm yet, it's like trying
to access user memory from a kernel thread... might happen to give
you -EFAULT on x86 but you are making pretty big assumptions on how
low level mm works in architectures...

Ben.

> 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 <tglx@xxxxxxxxxxxxx>
> Acked-by: Ingo Molnar <mingo@xxxxxxx>
> Cc: Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>
> Cc: Riku Voipio <riku.voipio@xxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
> ---
>  include/linux/futex.h |    1 +
>  kernel/futex.c        |   38 ++++++++++++++++++++++++++++++++++----
>  kernel/futex_compat.c |    9 +++++++++
>  3 files changed, 44 insertions(+), 4 deletions(-)
> 
> --- a/include/linux/futex.h
> +++ b/include/linux/futex.h
> @@ -153,6 +153,7 @@ union futex_key {
>  #ifdef CONFIG_FUTEX
>  extern void exit_robust_list(struct task_struct *curr);
>  extern void exit_pi_state_list(struct task_struct *curr);
> +extern int futex_cmpxchg_enabled;
>  #else
>  static inline void exit_robust_list(struct task_struct *curr)
>  {
> --- 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)
>  
>  /*
> @@ -466,6 +468,8 @@ void exit_pi_state_list(struct task_stru
>  	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
> @@ -1854,6 +1858,8 @@ 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:
>  	 */
> @@ -1878,6 +1884,9 @@ sys_get_robust_list(int pid, struct robu
>  	struct robust_list_head __user *head;
>  	unsigned long ret;
>  
> +	if (!futex_cmpxchg_enabled)
> +		return -ENOSYS;
> +
>  	if (!pid)
>  		head = current->robust_list;
>  	else {
> @@ -1980,6 +1989,9 @@ void exit_robust_list(struct task_struct
>  	unsigned long futex_offset;
>  	int rc;
>  
> +	if (!futex_cmpxchg_enabled)
> +		return;
> +
>  	/*
>  	 * Fetch the list head (which was registered earlier, via
>  	 * sys_set_robust_list()):
> @@ -2034,7 +2046,7 @@ void exit_robust_list(struct task_struct
>  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;
>  
> @@ -2062,13 +2074,16 @@ long do_futex(u32 __user *uaddr, int op,
>  		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;
> @@ -2123,8 +2138,23 @@ static struct file_system_type futex_fs_
>  
>  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);
> --- a/kernel/futex_compat.c
> +++ b/kernel/futex_compat.c
> @@ -54,6 +54,9 @@ void compat_exit_robust_list(struct task
>  	compat_long_t futex_offset;
>  	int rc;
>  
> +	if (!futex_cmpxchg_enabled)
> +		return;
> +
>  	/*
>  	 * Fetch the list head (which was registered earlier, via
>  	 * sys_set_robust_list()):
> @@ -115,6 +118,9 @@ asmlinkage long
>  compat_sys_set_robust_list(struct compat_robust_list_head __user *head,
>  			   compat_size_t len)
>  {
> +	if (!futex_cmpxchg_enabled)
> +		return -ENOSYS;
> +
>  	if (unlikely(len != sizeof(*head)))
>  		return -EINVAL;
>  
> @@ -130,6 +136,9 @@ compat_sys_get_robust_list(int pid, comp
>  	struct compat_robust_list_head __user *head;
>  	unsigned long ret;
>  
> +	if (!futex_cmpxchg_enabled)
> +		return -ENOSYS;
> +
>  	if (!pid)
>  		head = current->compat_robust_list;
>  	else {
> 

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