Re: [3.11-rc1] CONFIG_DEBUG_MUTEXES=y using gcc 3.x makesunbootablekernel.

[Peter Zijlstra]

On Mon, Sep 09, 2013 at 08:56:53PM +0900, Tetsuo Handa wrote:
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Mon, 9 Sep 2013 20:48:13 +0900
> Subject: [PATCH] mutex: Avoid gcc version dependent __builtin_constant_p() usage.
> 
> Commit 040a0a37 "mutex: Add support for wound/wait style locks" used
> "!__builtin_constant_p(p == NULL)" but gcc 3.x cannot handle such expression
> correctly, leading to boot failure when built with CONFIG_DEBUG_MUTEXES=y.
> 
> Fix it by explicitly passing a bool which tells whether p != NULL or not.
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxx> [3.11+]
> ---
>  kernel/mutex.c |   32 ++++++++++++++++----------------
>  1 files changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/mutex.c b/kernel/mutex.c
> index a52ee7bb..a2b80f1 100644
> --- a/kernel/mutex.c
> +++ b/kernel/mutex.c
> @@ -408,7 +408,7 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock,
>  static __always_inline int __sched
>  __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		    struct lockdep_map *nest_lock, unsigned long ip,
> -		    struct ww_acquire_ctx *ww_ctx)
> +		    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
>  {
>  	struct task_struct *task = current;
>  	struct mutex_waiter waiter;
> @@ -448,7 +448,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		struct task_struct *owner;
>  		struct mspin_node  node;
>  
> -		if (!__builtin_constant_p(ww_ctx == NULL) && ww_ctx->acquired > 0) {
> +		if (use_ww_ctx && ww_ctx->acquired > 0) {
>  			struct ww_mutex *ww;
>  
>  			ww = container_of(lock, struct ww_mutex, base);
> @@ -478,7 +478,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		if ((atomic_read(&lock->count) == 1) &&
>  		    (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
>  			lock_acquired(&lock->dep_map, ip);
> -			if (!__builtin_constant_p(ww_ctx == NULL)) {
> +			if (use_ww_ctx) {
>  				struct ww_mutex *ww;
>  				ww = container_of(lock, struct ww_mutex, base);
>  
> @@ -548,7 +548,7 @@ slowpath:
>  			goto err;
>  		}
>  
> -		if (!__builtin_constant_p(ww_ctx == NULL) && ww_ctx->acquired > 0) {
> +		if (use_ww_ctx && ww_ctx->acquired > 0) {
>  			ret = __mutex_lock_check_stamp(lock, ww_ctx);
>  			if (ret)
>  				goto err;
> @@ -568,7 +568,7 @@ done:
>  	mutex_remove_waiter(lock, &waiter, current_thread_info());
>  	mutex_set_owner(lock);
>  
> -	if (!__builtin_constant_p(ww_ctx == NULL)) {
> +	if (use_ww_ctx) {
>  		struct ww_mutex *ww = container_of(lock,
>  						      struct ww_mutex,
>  						      base);
> @@ -618,7 +618,7 @@ mutex_lock_nested(struct mutex *lock, unsigned int subclass)
>  {
>  	might_sleep();
>  	__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE,
> -			    subclass, NULL, _RET_IP_, NULL);
> +			    subclass, NULL, _RET_IP_, NULL, 0);
>  }
>  
>  EXPORT_SYMBOL_GPL(mutex_lock_nested);

This is a sad patch, but provided it actually generates similar code I
suppose its the best we can do bar whole sale deprecating gcc-3.
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