Re: [patch, -rc5-mm3] better lock debugging: remove mutex deadlock checking code

[Andy Whitcroft <apw@shadowen.org>]

Ingo Molnar wrote:
> * Randy.Dunlap <rdunlap@xenotime.net> wrote:
> 
> 
>>BUG: unable to handle kernel paging request at virtual address 22222232
> 
> 
> ok, this was a big thinko on my part, and it was right before our eyes. 
> Mutex deadlock checking relied on the 'big mutex debugging lock', but 
> that one is gone now - so mutex deadlock checking became racy (as your 
> crashes nicely pinpointed that). The races are more likely with an 
> increasing number of CPUs.
> 
> so the patch below finishes the cleanup i started: it removes deadlock 
> checking from the mutex code and lets the lock validator do that. This 
> should also be (much) faster on SMP, because the lock validator is 
> lockless in the fastpath. (if CONFIG_DEBUG_LOCKDEP is disabled)
> 
> 	Ingo
> 
> ----------------
> Subject: better lock debugging: remove mutex deadlock checking code
> From: Ingo Molnar <mingo@elte.hu>
> 
> with the lock validator we detect mutex deadlocks (and more), the mutex
> deadlock checking code is both redundant and slower. So remove it.
> 
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> ---
>  kernel/mutex-debug.c |  126 ---------------------------------------------------
>  lib/Kconfig.debug    |    8 ---
>  2 files changed, 1 insertion(+), 133 deletions(-)
> 
> Index: linux/kernel/mutex-debug.c
> ===================================================================
> --- linux.orig/kernel/mutex-debug.c
> +++ linux/kernel/mutex-debug.c
> @@ -23,128 +23,6 @@
>  
>  #include "mutex-debug.h"
>  
> -static void printk_task(struct task_struct *p)
> -{
> -	if (p)
> -		printk("%16s:%5d [%p, %3d]", p->comm, p->pid, p, p->prio);
> -	else
> -		printk("<none>");
> -}
> -
> -static void printk_ti(struct thread_info *ti)
> -{
> -	if (ti)
> -		printk_task(ti->task);
> -	else
> -		printk("<none>");
> -}
> -
> -static void printk_lock(struct mutex *lock, int print_owner)
> -{
> -#ifdef CONFIG_PROVE_MUTEX_LOCKING
> -	printk(" [%p] {%s}\n", lock, lock->dep_map.name);
> -#else
> -	printk(" [%p]\n", lock);
> -#endif
> -
> -	if (print_owner && lock->owner) {
> -		printk(".. held by:  ");
> -		printk_ti(lock->owner);
> -		printk("\n");
> -	}
> -}
> -
> -static void report_deadlock(struct task_struct *task, struct mutex *lock,
> -			    struct mutex *lockblk)
> -{
> -	printk("\n%s/%d is trying to acquire this lock:\n",
> -		current->comm, current->pid);
> -	printk_lock(lock, 1);
> -	debug_show_held_locks(current);
> -
> -	if (lockblk) {
> -		printk("but %s/%d is deadlocking current task %s/%d!\n\n",
> -			task->comm, task->pid, current->comm, current->pid);
> -		printk("\n%s/%d is blocked on this lock:\n",
> -			task->comm, task->pid);
> -		printk_lock(lockblk, 1);
> -
> -		debug_show_held_locks(task);
> -
> -		printk("\n%s/%d's [blocked] stackdump:\n\n",
> -			task->comm, task->pid);
> -		show_stack(task, NULL);
> -	}
> -
> -	printk("\n%s/%d's [current] stackdump:\n\n",
> -		current->comm, current->pid);
> -	dump_stack();
> -	debug_show_all_locks();
> -	printk("[ turning off deadlock detection. Please report this. ]\n\n");
> -	local_irq_disable();
> -}
> -
> -/*
> - * Recursively check for mutex deadlocks:
> - */
> -static int check_deadlock(struct mutex *lock, int depth, struct thread_info *ti)
> -{
> -	struct mutex *lockblk;
> -	struct task_struct *task;
> -
> -	if (!debug_locks)
> -		return 0;
> -
> -	ti = lock->owner;
> -	if (!ti)
> -		return 0;
> -
> -	task = ti->task;
> -	/*
> -	 * In the PROVE_MUTEX_LOCKING we are tracking all held
> -	 * locks already, which allows us to optimize this:
> -	 */
> -#ifdef CONFIG_PROVE_MUTEX_LOCKING
> -	if (!task->lockdep_depth)
> -		return 0;
> -#endif
> -	lockblk = NULL;
> -	if (task->blocked_on)
> -		lockblk = task->blocked_on->lock;
> -
> -	/* Self-deadlock: */
> -	if (current == task) {
> -		debug_locks_off();
> -		if (depth)
> -			return 1;
> -		printk("\n==========================================\n");
> -		printk(  "[ BUG: lock recursion deadlock detected! |\n");
> -		printk(  "------------------------------------------\n");
> -		report_deadlock(task, lock, NULL);
> -		return 0;
> -	}
> -
> -	/* Ugh, something corrupted the lock data structure? */
> -	if (depth > 20) {
> -		debug_locks_off();
> -		printk("\n===========================================\n");
> -		printk(  "[ BUG: infinite lock dependency detected!? |\n");
> -		printk(  "-------------------------------------------\n");
> -		report_deadlock(task, lock, lockblk);
> -		return 0;
> -	}
> -
> -	/* Recursively check for dependencies: */
> -	if (lockblk && check_deadlock(lockblk, depth+1, ti)) {
> -		printk("\n============================================\n");
> -		printk(  "[ BUG: circular locking deadlock detected! ]\n");
> -		printk(  "--------------------------------------------\n");
> -		report_deadlock(task, lock, lockblk);
> -		return 0;
> -	}
> -	return 0;
> -}
> -
>  /*
>   * Must be called with lock->wait_lock held.
>   */
> @@ -178,9 +56,7 @@ void debug_mutex_add_waiter(struct mutex
>  			    struct thread_info *ti)
>  {
>  	SMP_DEBUG_WARN_ON(!spin_is_locked(&lock->wait_lock));
> -#ifdef CONFIG_DEBUG_MUTEX_DEADLOCKS
> -	check_deadlock(lock, 0, ti);
> -#endif
> +
>  	/* Mark the current thread as blocked on the lock: */
>  	ti->task->blocked_on = waiter;
>  	waiter->lock = lock;
> Index: linux/lib/Kconfig.debug
> ===================================================================
> --- linux.orig/lib/Kconfig.debug
> +++ linux/lib/Kconfig.debug
> @@ -164,14 +164,6 @@ config DEBUG_MUTEX_ALLOC
>  	 (kfree(), kmem_cache_free(), free_pages(), vfree(), etc.),
>  	 or whether there is any lock held during task exit.
>  
> -config DEBUG_MUTEX_DEADLOCKS
> -	bool "Detect mutex related deadlocks"
> -	default y
> -	depends on DEBUG_MUTEXES
> -	help
> -	 This feature will automatically detect and report mutex related
> -	 deadlocks, as they happen.
> -
>  config DEBUG_RT_MUTEXES
>  	bool "RT Mutex debugging, deadlock detection"
>  	default y

I'll shove this one in for testing too.  Results on TKO as I have them.

-apw