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From: Robert Watson <rwatson@FreeBSD.org>
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To: Choupani <Choupani@gmail.com>
Subject: Re: protect common resources in kernel
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On Sun, 2 Dec 2012, Choupani wrote:

> I'm working on kernel in FreeBSD-9. I need to protect a 
> common resource (for example a queue). 
> There are 4 points for access (read/write) this common resource as bellows:
> 1. ether_input() – hardware interrupt
> 2. ip_input() & ip_output() – software interrupt
> 3. dev_ioctl() – local io control in our own kernel module
> 4. another kernel thread
>
> Which scenario is proper to use for this purpose:
>
> 1. kernel mutex (MTX_DEF)
> 2. kernel mutex (MTX_SPIN)
> 3. kernel share/exclusive lock
> 4. kernel reader/writer lock

Hi Choupani:

Assuming you are not accessing the resource from a low-level interrupt handler 
("filter") or within the scheduler, your best bets are (1) or (4), depending 
on whether you think you will benefit from read-locking as opposed to just 
write-locking.  (2) should be avoided unless in the low-level 
interrupt/scheduler context, as it takes additional overhead (disabling 
interrupts, etc), and (3) can't be used in contexts were unbounded sleeping 
isn't allowed (e.g., from ithreads, within most parts of the lower network 
stack).

Robert
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