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From: Roger Larsson <roger.larsson@norran.net>
Date: Sat, 25 Nov 2000 16:07:25 +0100
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To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nigel Gamble <nigel@nrg.org>
Subject: *_trylock return on success?
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Hi,

Background information:
 compiled and tested a test11 with the Montavista preemptive patch.
 After pressing Magic-SysRq-M all processes that tried to do IO hung in 'D'
 Last message "Buffer memory ..."
 Pressing Magic-SysRq-M again, all hung processes continued...

Checking the patch it looks like this

 	printk("Buffer memory:   %6dkB\n",
 			atomic_read(&buffermem_pages) << (PAGE_SHIFT-10));

-#ifdef CONFIG_SMP /* trylock does nothing on UP and so we could deadlock */
-	if (!spin_trylock(&lru_list_lock))
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
+	if (!mutex_trylock(&lru_list_mtx))
 		return;
 	for(nlist = 0; nlist < NR_LIST; nlist++) {

Ok, so I run some more code now than before (UP system with PREEMPT).
mutex_trylock is defined as:

+#define mutex_trylock(x) down_trylock(x)

Noticed that if the spin_trylock returns 0 on success, I will get the 
behavior I see.
  Not printing buffer info first time.
  Holding the lock - stopping other fs processes.
  Failing the mutex_trylock next attempt, interprete as success
  - continuing and printing the buffer info.
  - finally release the mutex

I removed the not (!) and now it works as expected.

Questions:
  What are _trylocks supposed to return?
  Does spin_trylock and down_trylock behave differently?
  Why isn't the expected return value documented?
  
/RogerL

-- 
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Home page:
  http://www.norran.net/nra02596/
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