linux-kernel - Re: [patch] x86: some lock annotations for user copy paths

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Date:	Wed, 10 Sep 2008 13:47:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] x86: some lock annotations for user copy paths


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Wed, 2008-09-10 at 13:37 +0200, Nick Piggin wrote:
> > copy_to/from_user and all its variants (except the atomic ones) can take a
> > page fault and perform non-trivial work like taking mmap_sem and entering
> > the filesyste/pagecache.
> > 
> > Unfortunately, this often escapes lockdep because a common pattern is to
> > use it to read in some arguments just set up from userspace, or write data
> > back to a hot buffer. In those cases, it will be unlikely for page reclaim
> > to get a window in to cause copy_*_user to fault.
> > 
> > With the new might_lock primitives, add some annotations to x86. I don't
> > know if I caught all possible faulting points (it's a bit of a maze, and I
> > didn't really look at 32-bit). But this is a starting point.
> > 
> > Boots and runs OK so far.
> 
> shouldn't some of that be conditional on pagefault_disable() 'n such?

i dont think so - those have their own special __atomic user-copy 
primitives which Nick didnt touch.

but i think it should be a single primitive sprinkled around, 
might_fault(), which would be something like:

> > +	might_sleep();
> > +	if (current->mm)
> > +		might_lock_read(&current->mm->mmap_sem);

that way it's a lot less visually intrusive as well.

	Ingo
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