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

[Manfred Spraul <manfreds@colorfullife.com>]

Andrea Arcangeli wrote:
> Look the swapout path. Without the big kernel lock you'll free vmas under
> swap_out().

I checked to code in mm/*.c, and it seems that reading the vma-list is
protected by either lock_kernel() [eg: swapper] or down(&mm->mmap_sem)
[eg: do_mlock].

But this means that both locks are required if you modify the vma list.
Single reader, multiple writer synchronization. Unusual, but interesting
:-)

Unfortunately, it seems that this is often ignored, eg. 

sys_mlock()->do_mlock()->merge_segments().
sys_brk()
sys_munmap() <<<<<< fixed by your patch.

It that correct?
Should I write a patch or is someone working on these problems?
How should we fix it?

a) the swapper calls down(&mm->mmap_sem), but I guess that would
lock-up.

b) everyone who changes the vma list calls lock_kernel().
I think it would be a bad thing to call lock_kernel() immediately in the
sys_??() function, I think we should hide the lock_kernel() call
somewhere
inside the vma-list code [add functions which modify the vma list, and
they call lock_kernel()].

--
	Manfred

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