Re: Multi-Threaded fork() correctness on Linux 2.4 & 2.6

[Hugh Dickins <hugh@veritas.com>]

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