Re: ipc,sem: sysv semaphore scalability

[Emmanuel Benisty]

On Sat, Mar 30, 2013 at 10:46 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Mar 29, 2013 at 8:02 PM, Emmanuel Benisty <benisty.e@xxxxxxxxx> wrote:
>>
>> Then I start building a random package and the problems start. They
>> may also happen without compiling but this seems to trigger the bug
>> quite quickly.
>
> I suspect it's about preemption, and the build just results in enough
> scheduling load that you start hitting whatever race there is.
>
>> Anyway, some progress here, I hope: dmesg seems to be
>> willing to reveal some secrets (using some pastebin service since this
>> is pretty big):
>>
>> https://gist.github.com/anonymous/5275120
>
> That looks like exactly the exit_sem() bug that Davidlohr was talking
> about, where the
>
>                 /* exit_sem raced with IPC_RMID, nothing to do */
>                 if (IS_ERR(sma))
>                         continue;
>
> should be moved to *before* the
>
>                 sem_lock(sma, NULL, -1);
>
> call. And apparently the bug I had found is already fixed in -next.

I just tried the 7 original patches + the 2 one liners from -next +
modified Linus' patch (attached) on the top of 3.9-rc4 using
PREEMPT_NONE and after moving sem_lock(sma, NULL, -1) as explained
above. I was building two packages at the same time, went away for 30
seconds, came back and everything froze as soon as I touched the
laptop's touchpad. Maybe a coincidence but anyway... Another shot in
the dark, I had this weird message when trying to build gcc:
semop(2): encountered an error: Identifier removed

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