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


On Mon, 19 Sep 2005, Hugh Dickins wrote:
>
> 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.

Hmm. But we do hold the mmap_sem for writing, and we flush before we
release it, so it should still be ok. The page fault case needs to get it
for reading anyway.

Yeah, the page_table_lock might make more sense, but I think the mmap_sem
thing works equally well.

Linus
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