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>Two Main Types of Kernel Locks: Spinlocks and Semaphores</H1
><P
> There are two main types of kernel locks. The fundamental type
is the spinlock
(<TT
CLASS="FILENAME"
>include/asm/spinlock.h</TT
>),
which is a very simple single-holder lock: if you can't get the
spinlock, you keep trying (spinning) until you can. Spinlocks are
very small and fast, and can be used anywhere.
</P
><P
> The second type is a semaphore
(<TT
CLASS="FILENAME"
>include/asm/semaphore.h</TT
>): it
can have more than one holder at any time (the number decided at
initialization time), although it is most commonly used as a
single-holder lock (a mutex). If you can't get a semaphore,
your task will put itself on the queue, and be woken up when the
semaphore is released. This means the CPU will do something
else while you are waiting, but there are many cases when you
simply can't sleep (see <A
HREF="x315.html"
>the Section called <I
>Things Which Sleep</I
> in the chapter called <I
>Common Techniques</I
></A
>), and so
have to use a spinlock instead.
</P
><P
> Neither type of lock is recursive: see
<A
HREF="x214.html"
>the Section called <I
>Deadlock: Simple and Advanced</I
> in the chapter called <I
>Common Techniques</I
></A
>.
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>Locks and Uniprocessor Kernels</H1
><P
> For kernels compiled without <SPAN
CLASS="SYMBOL"
>CONFIG_SMP</SPAN
>, spinlocks
do not exist at all. This is an excellent design decision: when
no-one else can run at the same time, there is no reason to
have a lock at all.
</P
><P
> You should always test your locking code with <SPAN
CLASS="SYMBOL"
>CONFIG_SMP</SPAN
>
enabled, even if you don't have an SMP test box, because it
will still catch some (simple) kinds of deadlock.
</P
><P
> Semaphores still exist, because they are required for
synchronization between <A
HREF="g388.html#GLOSS-USERCONTEXT"
><I
CLASS="FIRSTTERM"
>user
contexts</I
></A
>, as we will see below.
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