linux-kernel - Re: profile code added to netif_receive

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Message-ID: <21348.1196136023@turing-police.cc.vt.edu>
Date:	Mon, 26 Nov 2007 23:00:23 -0500
From:	Valdis.Kletnieks@...edu
To:	kernel coder <lhrkernelcoder@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: profile code added to netif_receive_skb function

On Sun, 25 Nov 2007 21:46:26 PST, kernel coder said:
> hi,
> 
> I have added some code to netif_receive_skb function.As linux kernel
> is multhreaded , so there is no gaurantee than mine code is completely
> executed without being disturbed by any other process .Timer interrupt
> handler is an example of code which might interrupt execution of mine
> code.

The trick is to write your code so it doesn't *matter* if other code runs.

For example - the timer interrupt almost certainly doesn't look at or modify
any of *your* code's variables.  So 98% of the kernel's code you don't
even have to *care* if it runs (as long as you aren't doing something
real-time or has similar response-time or throughput constraints).

And if you are worried about that other 2%, where related code, for example the
IRQ handler for a network interface, may have to look at and/or modify some of
your variables, that's when you should be using appropriate locking - there's
mutexes, semaphores, the whole RCU family, and more - none of which I'll
attempt to explain, because I'm not all that good at that stuff.

Basic rule of thumb - if you have something that will break if two things
access it at the same time, put a lock around it, so they take turns.

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