linux-kernel - Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] : ima-appraisal patches)

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Date:	Fri, 20 Apr 2012 20:04:18 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	linux-fsdevel@...r.kernel.org, James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	David Safford <safford@...ux.vnet.ibm.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] :
 ima-appraisal patches)

On Fri, Apr 20, 2012 at 11:54:01AM -0700, Hugh Dickins wrote:
> I can see that the discussion has since moved on quite a way from here.
> 
> But it looks to me fairly easy for mm to stop doing fput() under mmap_sem.
> 
> That's already the case when exiting (no mmap_sem held), and shouldn't add
> observable cost when unmapping (we already work on a chain of vmas to be
> freed, and when unmapping that chain will usually just be of one: shouldn't
> matter to defer a final pass until after mmap_sem is dropped).  Unless I'm
> mistaken, the fput() buried in vma_adjust() can never be a final fput.
> 
> Is it worth my trying to implement that?  Or do you see an immediate
> gotcha that I'm missing?  Or are you happy enough with your deferred
> fput() ideas, that it would be a waste of time to rearrange the mm end?

Deferring the final pass after dropping ->mmap_sem is going to be
interesting; what would protect ->vm_next on those suckers?  Locking
rules are already bloody complicated in mm/*; this will only add more
subtle fun.  Note that right now both the dissolution of ->vm_next
list and freeing of VMAs happen under ->mmap_sem; changing that might
cost us a lot of headache...
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