Re: locking question: do_mmap(), do_munmap()

[Alexander Viro <viro@math.psu.edu>]

On Mon, 11 Oct 1999, Stephen C. Tweedie wrote:

> Hi,
> 
> On Mon, 11 Oct 1999 20:02:40 +0200, Manfred Spraul
> <manfreds@colorfullife.com> said:
> 
> > What about something like a rw-semaphore which protects the vma list:
> > vma-list modifiers [ie merge_segments(), insert_vm_struct() and
> > do_munmap()] grab it exclusive, swapper grabs it "shared, starve
> > exclusive".
> 
> Deadlock.  Process A tries to do an mmap on mm A, gets the exclusive
> lock, tries to swap out from process B, and grabs mm B's shared lock.
> Process B in the mean time is doing the same thing and has an exclusive
> lock on mm B, and is trying to share-lock A.  Whoops.

<looking at the places in question>
insert_vm_struct doesn't allocate anything.
Ditto for merge_segments
In do_munmap() the area that should be protected (ripping the vmas from
the list) doesn't allocate anything too.
In the swapper we are protected from recursion, aren't we?

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