linux-kernel - spinlock which can morph into a mutex

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Date:	Fri, 18 Dec 2009 15:30:35 +0100
From:	Miquel van Smoorenburg <miquels@...tron.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, miquels@...tron.nl
Subject: spinlock which can morph into a mutex

I'm trying to implement a dynamically resizable hashtable, and
I have found that after resizing the table I need to call
synchronize_rcu() and finish up before letting other writers
(inserts, deletes) access the table.

Ofcourse during the hashtable update a spinlock is held to
exclude the other writers. But I cannot hold this spinlock over
synchronize_rcu(), yet the other writers still need to be excluded.

So I probably need a mutex instead of a spinlock, but I want to
keep minimal overhead for the common case (when no resizing is in
progress).  I think I need a spinlock that can morph into a mutex ..

I was thinking about using something like the code below.
It is sortof like a spinlock, but it's ofcourse less fair
than actual ticketed spinlocks.

I'm working off 2.6.27 at the moment, but I noticed that in
2.6.28 adaptive spinning was introduced for mutexes. Is the
approach below still worth it with adaptive spinning or could
I just convert the spinlocks to mutexes with minimal extra overhead ?

Example code:

int real_mutex_lock = 0; // can use int since mutex ops are barriers
struct mutex mutex;

// 1. used instead of spinlock() [common case]
while (mutex_trylock(&mutex) == 0) {
	if (real_mutex_lock) {
		mutex_lock(&mutex);
		break;
	}
}
.. have lock, do work
mutex_unlock(&mutex);

// 2. When we want to lock and be able to sleep [seldomly used]
mutex_lock(&mutex);
real_mutex_lock = 1;
smp_wmb();

.. do work ..
real_mutex_lock = 0;
mutex_unlock(&mutex);

Mike.
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