Messages in this thread | | | Date | Wed, 9 Jun 2010 08:39:00 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately. |
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On Wed, 9 Jun 2010, Ingo Molnar wrote:
> > * Salman <sqazi@google.com> wrote: > > > > A program that repeatedly forks and waits is susceptible to having the > > same pid repeated, especially when it competes with another instance of the > > same program. This is really bad for bash implementation. Furthermore, many shell > > scripts assume that pid numbers will not be used for some length of time. > > > > Thanks to Ted Tso for the key ideas of this implementation. > > Looks rather interesting. (Cleanliness-wise i'd suggest to split out the while > loop into a helper function.)
I have to say that usually I can look at a patch and see what it does.
This time I had absolutely no clue.
There was a whole big context missing: that original load of "last_pid" into "last" at the top of the function, far outside the patch. And in this case I don't think splitting out the trivial cmpxchg() loop would help that problem - that would just make the "load original" part of the big picture be even further away from the final "replace if same" part, and I think _that_ is a much more critical part of the subtleties there.
So I had to read the patch _and_ go read the code it patched, in order to at all understand what it did. I think the patch explanation should have done it, and I also think this would need a bit comment at the top.
[ In fact, I'd argue that the _old_ code would have needed a big comment at the top about last_pid usage, but i somebody had done that, they'd probably also have seen the race while explaning how the code worked ;]
So having looked at the patch and the code, I agree with the patch, but I'd like some more explanation to go with it.
[ Or Ted's version: as mentioned, I don't think the complexity is actually in the final cmpxchg loop itself, but in the bigger picture, so I don't think the differences between Ted's and Salman's versions are that big ]
Linus
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