Messages in this thread |  | | From | (Eric W. Biederman) | Date | Mon, 10 Aug 2009 12:21:44 -0700 | Subject | Re: Possible memory leak via alloc_pid() |
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Catalin Marinas <catalin.marinas@arm.com> writes:
> On Sun, 2009-08-02 at 18:44 -0700, Eric W. Biederman wrote: >> Hmm. I'm starting to wonder if kmemleak is right. I don't know how >> it works but something about the way pids are used might be confusing it. > > It could as well be a false positive but I can't find its source. > > Basically, the pid structure for the dead Xorg is still allocated > minutes after Xorg died with a pid->count of 2. Kmemleak scans the data > and bss sections, task stacks and most of the allocated objects (which > are not reported as leaks) but cannot find a pointer to this pid > structure (or anywhere inside it like pid->number.pid_chain). > > The supposedly leaked pid structure also have pid_chain.pprev == > LIST_POISON2 which means that it was already removed from the pid_hash > (this block of memory is scanned by kmemleak anyway). > > The free_pid() function was also called on this object according to the > pid->rcu values but put_pid() couldn't free it because of pid->count. > > If this structure in not on pid_hash, is there any other place where its > pointer may be stored for a long time? Otherwise it looks like a real > leak (though not a big one).
It depends on how it is used. Pids not on the pid_hash are perfectly fine. We use these kinds of long standing pid references to prevent any chance that pid rollover would be a problem. There are corner cases in the SIGIO path and a few other places where we have these kind of long standing pid references.
Perhaps it comes from the tty switching code?
> I'll do more tests in the next few days as suggested by Oleg.
Thanks, and thanks for spotting into and looking into this.
Eric
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