[PATCH 0/2] ipc/sem.c: sem_lock fixes

[Manfred Spraul]

Hi Andrew, Hi Peter,

next version of the sem_lock() fixes / improvement:
The patches are now vs. tip.

Patch 1 is ready for merging, patch 2 is new and for discussion.

Patch 1 fixes the race that was found by Felix.
It also adds smp_mb() to fully synchronize

        WRITE_ONCE(status, 1);
        <<< smp_mb();
        spin_unlock_wait();

vs.
        spin_lock();
        <<< smp_mb();
        READ_ONCE(status);

Patch 2 tries to close a performance regression:
        sem_wait_array() must perform a spin_unlock_wait() for every
        semaphore in the array. If the array is large, then this is
        slow.

        Two open points:
        - Do we need it? Factor 20 improvement is nice, but is the
          test realistic?
        - COMPLEX_MODE_ENTER=10 is a magic number, without any
          rational. (2+sem_nsems/128) would be another option,
          but also arbitrary.
 
        Test:
         https://github.com/manfred-colorfu/ipcscale
         # ./sem-scalebench -o 4 -f -m 0 -h 4 -i 1 -p 2 -c 8 -t 5 -d 500
          Before: 214732
          After: 4008635

--
        Manfred