linux-kernel - new implementation of mutex

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Date:	Mon, 1 Jun 2009 14:49:09 +0800
From:	谢纲 <xiegang112@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: new implementation of mutex

There is new implementaion in newer kernel (my kernel version is
2.6.27). Compared to the old implementaion which uses binary
semaphore, there are some new features:
 - 'struct mutex' semantics are well-defined and are enforced if
   CONFIG_DEBUG_MUTEXES is turned on. Semaphores on the other hand have
   virtually no debugging code or instrumentation. The mutex subsystem
   checks and enforces the following rules:

   * - only one task can hold the mutex at a time
   * - only the owner can unlock the mutex
   * - multiple unlocks are not permitted
   * - recursive locking is not permitted
   * - a mutex object must be initialized via the API
   * - a mutex object must not be initialized via memset or copying
   * - task may not exit with mutex held
   * - memory areas where held locks reside must not be freed
   * - held mutexes must not be reinitialized
   * - mutexes may not be used in hardware or software interrupt
   *   contexts such as tasklets and timers

But in my test, I try to lock mutex in one thread, and unlock it in
the other thread. There is nothing wrong happens. It works just like
semaphore. I have had CONFIG_DEBUG_MUTEXES turned on.
The two threads are mostly like this:
struct mutex mutex;

static int mysthread1(void * data){
  int i;
  i= 5;
  while(i -- > 0){
                mutex_lock(&mutex);
    printk("this is thread1\n");
    msleep(5000);
  }
        return 0;
}

static int mysthread2(void * data){
  int i;
  i= 5;
  while(i -- > 0){
    printk("this is thread2\n");
    msleep(5000);
                mutex_lock(&mutex);
  }
        return 0;
}

I debug it and find the debug_locks = 0.  Is this why there is no
warning message and how turn it on? (I also had CONFIG_LOCKDEP_SUPPORT
on.)


-- 
Xie Gang
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