[PATCH] 2.4-ac: migration_thread locking fix

[Robert Love <rml@tech9.net>]

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Alan,

Anton Blanchard reported a lock vs. interrupt-off ordering bug in
migration_thread.  The bug is already fixed in 2.5.  A patch is attached
to fix O(1) in your tree.

See the comments above double_rq_lock and double_rq_unlock (which,
sadly, I wrote)!  Without this patch there is a possibility to hang,
which Anton observed.

Patch is against 2.4.19-pre8-ac1, please apply.

	Robert Love


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diff -urN linux-2.4.19-pre8-ac1/kernel/sched.c linux/kernel/sched.c
--- linux-2.4.19-pre8-ac1/kernel/sched.c	Wed May  8 12:03:14 2002
+++ linux/kernel/sched.c	Sat May 11 09:29:55 2002
@@ -1640,8 +1640,8 @@
 		local_irq_save(flags);
 		double_rq_lock(rq_src, rq_dest);
 		if (p->cpu != cpu_src) {
-			local_irq_restore(flags);
 			double_rq_unlock(rq_src, rq_dest);
+			local_irq_restore(flags);
 			goto repeat;
 		}
 		if (rq_src == rq) {
@@ -1651,8 +1651,8 @@
 				activate_task(p, rq_dest);
 			}
 		}
-		local_irq_restore(flags);
 		double_rq_unlock(rq_src, rq_dest);
+		local_irq_restore(flags);
 
 		up(&req->sem);
 	}