<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML
><HEAD
><TITLE
>Locking Between Bottom Halves</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
REL="HOME"
TITLE="Unreliable Guide To Locking"
HREF="book1.html"><LINK
REL="UP"
TITLE="Two Main Types of Kernel Locks: Spinlocks and Semaphores"
HREF="c88.html"><LINK
REL="PREVIOUS"
TITLE="Locking Between User Context and Tasklets/Soft IRQs"
HREF="x138.html"><LINK
REL="NEXT"
TITLE="Locking Between Tasklets"
HREF="x153.html"></HEAD
><BODY
CLASS="SECT1"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#0000FF"
VLINK="#840084"
ALINK="#0000FF"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Unreliable Guide To Locking</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="x138.html"
ACCESSKEY="P"
><<< Previous</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Two Main Types of Kernel Locks: Spinlocks and Semaphores</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="x153.html"
ACCESSKEY="N"
>Next >>></A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LOCK-BH"
></A
>Locking Between Bottom Halves</H1
><P
> Sometimes a bottom half might want to share data with
another bottom half (especially remember that timers are run
off a bottom half).
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCK-BH-SAME"
></A
>The Same BH</H2
><P
> Since a bottom half is never run on two CPUs at once, you
don't need to worry about your bottom half being run twice
at once, even on SMP.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCK-BH-DIFFERENT"
></A
>Different BHs</H2
><P
> Since only one bottom half ever runs at a time once, you
don't need to worry about race conditions with other bottom
halves. Beware that things might change under you, however,
if someone changes your bottom half to a tasklet. If you
want to make your code future-proof, pretend you're already
running from a tasklet (see below), and doing the extra
locking. Of course, if it's five years before that happens,
you're gonna look like a damn fool.
</P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="x138.html"
ACCESSKEY="P"
><<< Previous</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="book1.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="x153.html"
ACCESSKEY="N"
>Next >>></A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Locking Between User Context and Tasklets/Soft IRQs</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="c88.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Locking Between Tasklets</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
