linux-kernel-driver-dev/spinlocks.html at master · txrxlabs/linux-kernel-driver-dev · GitHub

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<html>
<font face="helvetica">
<title>Spin Locks</title>
<body>
<p align=center><font size=+5>Spin Locks</font>
<hr>
<a href="contents.html">Contents</a>
<font size=+3>
<p>Spin locks are one of the main synchronization methods
used within the kernel.  Read Documentation/spinlocks.txt.
(You will probably be confused.  This is normal.)
<p>There are three main variants of the spin lock
functions:
<ul>
<li>
<pre>
	spin_lock_irqsave(&lock, flags);
	...
	spin_unlock_irqrestore(&lock, flags);
</pre>
Spin_lock_irqsave() records the current cpu flags (mainly whether interrupts on the
local cpu are currently disabled or not), disables interrupts on the local cpu, and
obtains the spin lock.  Spin_lock_irqrestore() releases the spin lock, restores the
cpu flags to the previous state (if interrupts were disabled/enabled prior to
spin_lock_irqsave, they are restored to disabled/enabled state.  Puts interrupts back
how they were prior to spin_lock_irqsave().  This is the safest way, as it always
does the right thing.
<li>
<pre>
	spin_lock_irq(&lock);
	...
	spin_unlock_irq(&lock);
</pre>
Spin_lock_irq disables interrupts and obtains the spin lock.  Prior interrupt
state is not recorded.  spin_lock_irq releases the spin lock and <em>unconditionally</em>
enables interupts.   You had better be damn sure interrupts are enabled if you
call spin_lock_irq(); / spin_unlock_irq();
<li>
<pre>
	spin_lock(&lock);
	...
	spin_unlock(&lock);
</pre>
<p>spin_lock() obtains the spin lock -- interrupt state is not affected.  Spin_unlock()
releases the spin_lock, interrupt state is not affected.
<p align=right><a href="spinlocks2.html">Next</a>
</font>
</body>
</html>