[PATCH] semaphore fairness patch against test11-pre6

David Mansfield
2000-11-18T01:32:11Z

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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();

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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();