Subject: Re: kernel stack size
From: Steven Rostedt <rostedt@goodmis.org>
To: Al Viro <viro@parcelfarce.linux.theplanet.co.uk>
Cc: ooyama eiichi <ooyama@tritech.co.jp>, cw@f00f.org,
       LKML <linux-kernel@vger.kernel.org>
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Organization: Kihon Technologies
Date: Sat, 02 Apr 2005 14:52:12 -0500
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On Sat, 2005-04-02 at 20:37 +0100, Al Viro wrote:
> On Sat, Apr 02, 2005 at 02:04:11PM -0500, Steven Rostedt wrote:
> > You can also use globally static variables too. But this makes for
> > non-reentry code.
> > 
> > Sometimes I don't feel that a kmalloc is worth it, and if the function
> > in question for the driver would seldom have problems with reentry, I
> > use a statically defined global, and protect it with spin_locks. If
> > these can also be used in interrupt context, you need to use the
> > spin_lock_irqsave variants.  But don't do this if the critical section
> > has long latencies.
> 
> ... and the first time copy_from_user() blocks under your spinlock
> you will get a nice shiny deadlock.

I forgot that he mentioned that this was for ioctls. I then use
semaphores if I need to access userspace. But if it just needs to modify
data around areas that only the kernel uses, without access to
userspace, than I use spinlocks.

I admit you really need to know what you're doing to use this method. If
I believe that a kmalloc would be too expensive, then I use the locking
of static variables. But each situation is different and I try to use
the best method for the occasion.

-- Steve


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