svn commit: r277643 - in head/sys: arm/arm dev/mem i386/i386 mips/mips sparc64/sparc64

Konstantin Belousov kostikbel at gmail.com
Sat Jan 24 15:42:51 UTC 2015


On Sat, Jan 24, 2015 at 07:56:37AM -0700, Ian Lepore wrote:
> On Sat, 2015-01-24 at 12:51 +0000, Konstantin Belousov wrote:
> > Author: kib
> > Date: Sat Jan 24 12:51:15 2015
> > New Revision: 277643
> > URL: https://svnweb.freebsd.org/changeset/base/277643
> > 
> > Log:
> >   Remove Giant from /dev/mem and /dev/kmem.  It is definitely not needed
> >   for i386, and from the code inspection, nothing in the
> >   arm/mips/sparc64 implementations depends on it.
> >   
> 
> I'm not sure I agree with that.  On arm the memrw() implementation uses
> a single statically-allocated page of kva space into which it maps each
> physical page in turn in the main loop.  What prevents preemption or
> multicore access to /dev/mem from trying to use that single page for
> multiple operations at once?

I see, thank you for noting this.

But, I do not think that Giant is a solution for the problem. uiomove()
call accesses userspace, which may fault and cause sleep. If the
thread sleeps, the Giant is automatically dropped, so there is no real
protection.

I think dump exclusive sx around whole memrw() should be enough.

I can revert the commit for now, or I can leave it as is while
writing the patch with sx and waiting for somebody review.  What
would you prefer ?

P.S. mips uses uiomove_fromphys(), avoiding transient mapping,
and sparc allocates KVA when needed.


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