gru: add comments raised in previous code reviews

Add comments from previous code reviews.  The comments help explain some
of the more esoteric aspects of the driver.

Move a free() to the other side of an unlock.
Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/misc/sgi-gru/grufault.c

@@ -733,6 +733,11 @@ long gru_get_gseg_statistics(unsigned long arg)
 	if (copy_from_user(&req, (void __user *)arg, sizeof(req)))
 		return -EFAULT;
 
+	/*
+	 * The library creates arrays of contexts for threaded programs.
+	 * If no gts exists in the array, the context has never been used & all
+	 * statistics are implicitly 0.
+	 */
 	gts = gru_find_lock_gts(req.gseg);
 	if (gts) {
 		memcpy(&req.stats, &gts->ustats, sizeof(gts->ustats));

drivers/misc/sgi-gru/grukservices.c

@@ -200,13 +200,15 @@ static int gru_free_kernel_contexts(void)
 		bs = gru_base[bid];
 		if (!bs)
 			continue;
+
+		/* Ignore busy contexts. Don't want to block here.  */
 		if (down_write_trylock(&bs->bs_kgts_sema)) {
 			kgts = bs->bs_kgts;
 			if (kgts && kgts->ts_gru)
 				gru_unload_context(kgts, 0);
-			kfree(kgts);
 			bs->bs_kgts = NULL;
 			up_write(&bs->bs_kgts_sema);
+			kfree(kgts);
 		} else {
 			ret++;
 		}