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From: John Baldwin <jhb@freebsd.org>
To: freebsd-current@freebsd.org
Subject: Re: copyin()/copyout() constraints ?
Date: Fri, 14 Jun 2013 12:07:29 -0400
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 (bigwig.baldwin.cx); Fri, 14 Jun 2013 12:14:53 -0400 (EDT)
Cc: Alfred Perlstein <bright@mu.org>, Luigi Rizzo <rizzo@iet.unipi.it>
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On Wednesday, June 12, 2013 2:36:52 pm Alfred Perlstein wrote:
> On 6/12/13 11:01 AM, Luigi Rizzo wrote:
> > hi,
> > is it possible to run copyin() or copyout() in one of these cases:
> > 1. while holding a spinlock
> > 2. while holding a regular mutex/lock
> > 3. while holding a read lock (on an RWLOCK or RMLOCK)
> > 4. while holding a write lock (on an RWLOCK or RMLOCK)
> >
> > I suspect #1 is forbidden, but am a bit unclear for the
> > other cases.
> No on all of the above unless the memory is wired.

If the memory is wired you can hold anything you want since it's basically a 
memcpy at that point.  Various sysctl handlers that need to hold locks while
writing things out use sysctl_wire_old_buffer() for exactly this reason.

-- 
John Baldwin