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

[Hugh Dickins <hugh@veritas.com>]

On Mon, 19 Sep 2005, Smarduch Mario-CMS063 wrote:

> MMU/Kernel experts,
>     
>     recently I've been involved in debugging MT forks on IA-64 the issues
> found were related to the way IA64 does its TLB invalidation. But there
> is still one  issue that appears to be Linux in general related, and it
> has to do with MT forks. 
>  
> For example a process has 3 threads T1, T2, T3.
>  
> 1 - T1 issues a fork()
>     - under page_table_lock write bit is reset in src and dest pte's
> 2 - T2 may have a TLB mis, the new write protected pte is inserted
>     (hw walker or sw tlb hdlr)
> 3 - T2 winds up in do_wp_page() the page is copied to a new one
> 4 - In the mean time T3 may be working off the same page, the
>     TLB invalidation (flush_tlb_mm()) has no occurred yet.
> 5 - Eventually TLB is globally flushed so threads will see the new pte.
> 6 - As a result the MT task may experience inconsisitent state.
>     - During 3 & 4. For example locks may be acquired by both
>       threads depending on the timing of the copy.

I do think you've hit upon something interesting here.  Though it's
not quite as you describe.  We don't have to wait for the flush_tlb_mm
to sort it out: the flush_tlb_page in ptep_establish in break_cow in
do_wp_page resolves the discrepancy much sooner.  But your point is,
that's already too late: T2 inserts a "smudged" copy of the page which
T3 was working on, it does not contain all the data T3 had written there.

> The TLB refill (hw/sw) work independently of the page_table_lock,
> it seems all threads should be forced to see the new pte
> before the new page is copied over by forcing the pte to 0 
> and allowing page_table_lock to synchronize the threads.

I don't get your pte to 0 suggestion.  What we seem to need is to
flush the TLB sooner (as well as in ptep_establish).  A first guess
is that do_wp_page needs (in these circumstances) to flush_tlb_page
before copying the page.

But this stuff is subtle, and TLB flushes shouldn't be added lightly.
I'm certainly not going to rush to propose the fix (and I could easily
be wrong in seeing the problem).  But perhaps others will be surer.

> I come from an SVR4 background and relatively new to
> Linux, any insights or corrections would be greatly appreciated.
> Please copy my email id as its to miss email on this list.

Welcome!

Hugh
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