Messages in this thread | | | Date | Wed, 9 Jun 2010 13:34:24 -0400 | From | tytso@mit ... | Subject | Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately. |
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On Wed, Jun 09, 2010 at 10:25:50AM -0700, Linus Torvalds wrote: > > > On Wed, 9 Jun 2010, Linus Torvalds wrote: > > > > Otherwise you have three threads, two of which pick the same pid (because > > the test-and-set isn't atomic), and a third of which picks a new one. > > In fact, I don't think you need three threads at all. It's perfectly ok to > just have two threads, and they'd both end up picking the same 'pid' > without the atomicity guarantees of that 'test_and_set()' bitmap access. > > And they'd both be perfectly fine setting last_pid to that (shared) pid if > I read that cmpxchg loop right. No?
Well, I was thinking about something like this:
while (1) { last = pid_ns->last_pid; pid = last + 1; if (pid >= pid_max) pid = RESERVED_PIDS; if (cmpxchg(&pid_ns->last_pid, last, pid) == last) return pid; }
Which I don't think is racy, unless I'm missing something. Both might end up picking the same pid, but only one will successfully set last_pid, and the other will just loop and try again.
There appears to be some interesting uses of the bitmap by find_ge_pid() and next_pidmap() that I haven't completely grokked yet, especially as to why they're needed, though. Assuming they are needed, we might end up needing the bitmap after all, though.
- Ted
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