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SubjectRE: Multi-Threaded fork() correctness on Linux 2.4 & 2.6
Date
mmap_sem is also acquired in 2.4 properly. 
It seemed a little bit too obvious.
Thanks for your help!

- mario

-----Original Message-----
From: Hugh Dickins [mailto:hugh@veritas.com]
Sent: Monday, September 19, 2005 3:14 PM
To: Linus Torvalds
Cc: Smarduch Mario-CMS063; linux-kernel@vger.kernel.org
Subject: Re: Multi-Threaded fork() correctness on Linux 2.4 & 2.6

On Mon, 19 Sep 2005, Linus Torvalds wrote:
>
> We hold the page_table_lock when doing the fork(), so T2 can't
> actually be copying the page until we've done the TLB flush, no? And
> once the TLB flush is done, all the writes by T3 should be in the
> page, so we copy the right thing at that point, and there is no consistency problems?

I was totally overlooking the page_table_lock during the fork.

But no matter, it's not good enough: src_mm->page_table_lock is acquired and dropped at the inner level, in copy_pte_range (looking at latest 2.6):
it cannot be held across allocating page tables for dst_mm.

So each time T1 drops it, there's a window for the T2 vs. T3 problem.
Yet we don't much want to flush TLB each time we leave copy_pte_range.

Hugh
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