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>Locking Only In User Context</H1
><P
> If you have a data structure which is only ever accessed from
user context, then you can use a simple semaphore
(<TT
CLASS="FILENAME"
>linux/asm/semaphore.h</TT
>) to protect it. This
is the most trivial case: you initialize the semaphore to the number
of resources available (usually 1), and call
<TT
CLASS="FUNCTION"
>down_interruptible()</TT
> to grab the semaphore, and
<TT
CLASS="FUNCTION"
>up()</TT
> to release it. There is also a
<TT
CLASS="FUNCTION"
>down()</TT
>, which should be avoided, because it
will not return if a signal is received.
</P
><P
> Example: <TT
CLASS="FILENAME"
>linux/net/core/netfilter.c</TT
> allows
registration of new <TT
CLASS="FUNCTION"
>setsockopt()</TT
> and
<TT
CLASS="FUNCTION"
>getsockopt()</TT
> calls, with
<TT
CLASS="FUNCTION"
>nf_register_sockopt()</TT
>. Registration and
de-registration are only done on module load and unload (and boot
time, where there is no concurrency), and the list of registrations
is only consulted for an unknown <TT
CLASS="FUNCTION"
>setsockopt()</TT
>
or <TT
CLASS="FUNCTION"
>getsockopt()</TT
> system call. The
<TT
CLASS="VARNAME"
>nf_sockopt_mutex</TT
> is perfect to protect this,
especially since the setsockopt and getsockopt calls may well
sleep.
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