[PATCH] semaphore fairness patch against test11-pre6

[David Mansfield]

Hi Linus et al,

I've applied your semaphore fairness patch (slightly fixed) below.  It
fixes my original bug report of vmstat, ps etc. stalls waiting for the
mmap_sem.  I can now run my memory 'hog' processes and actually see
vmstat update every second even under heavy memory pressure.  More
importantly, ps works so I can find the pid to kill.  I'm no expert in
checking for races, but I went over all (I think) the 2 process cases as
well as I could and they seem to look ok to me, but what do I know.  I
know someone else reported it didn't fix the problem, but perhaps that's
some other issue.

I ran many 'ps' (20?) in the background trying to simulate many process
contention, and everything still worked fine.  I've run some other
stress tests too (dbench, my own I/O throughput test etc) and so far
all's well (famous last words).

If you can find the time to check this out more completely, I recommend
it, because it seems like a great improvement to be able to accurately
see vmstat numbers in times of system load.  I hope the other side
effects are beneficial as well :-)

The change to the patch was that you had 'if (sleepers > 1)' when
obviously you meant 'if (sem->sleepers > 1)'...

Here's your patch again (also attached in case of mangling):

--- linux/arch/i386/kernel/semaphore.c  2000/11/16 19:58:26     1.3
+++ linux/arch/i386/kernel/semaphore.c  2000/11/17 23:12:48
@@ -64,6 +64,14 @@
 
        spin_lock_irq(&semaphore_lock);
        sem->sleepers++;
+
+       /*
+        * Are there other people waiting for this?
+        * They get to go first.
+        */
+       if (sem->sleepers > 1)
+               goto inside;
+
        for (;;) {
                int sleepers = sem->sleepers;
 
@@ -76,6 +84,7 @@
                        break;
                }
                sem->sleepers = 1;      /* us - see -1 above */
+inside:
                spin_unlock_irq(&semaphore_lock);
 
                schedule();
Index: linux/arch/i386/kernel/semaphore.c
===================================================================
RCS file: /home/kernel/cvs_master/linux/arch/i386/kernel/semaphore.c,v
retrieving revision 1.3
diff -u -r1.3 semaphore.c
--- linux/arch/i386/kernel/semaphore.c  2000/11/16 19:58:26     1.3
+++ linux/arch/i386/kernel/semaphore.c  2000/11/17 23:12:48
@@ -64,6 +64,14 @@
 
        spin_lock_irq(&semaphore_lock);
        sem->sleepers++;
+
+       /*
+        * Are there other people waiting for this?
+        * They get to go first.
+        */
+       if (sem->sleepers > 1)
+               goto inside;
+
        for (;;) {
                int sleepers = sem->sleepers;
 
@@ -76,6 +84,7 @@
                        break;
                }
                sem->sleepers = 1;      /* us - see -1 above */
+inside:
                spin_unlock_irq(&semaphore_lock);
 
                schedule();