futex_atomic_cmpxchg_inatomic usage

RFC PATCH New Darren Hart 1 participant 1 message Aug 3, 2006

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Darren Hart

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2006-08-03 03:35 UTC #0 lore raw

RFC PATCH 1/1 futex_atomic_cmpxchg_inatomic usage

While reviewing futex.c re Esben's rt_mutex_next_owner() serialization posting, I had a question regarding the use of

futex_atomic_cmpxchg_inatomic()

In all but two places in futex.c we wrap that call with (inc|dec)_preempt_count() calls. Nothing stood out to me that would account for the difference. Can some comment on why we don't have to wrap the call in those two places (see patch for locations). Just in case this happens to be a bug, I've included a patch to fix it.

kernel/futex.c +5 -1

Index: 2.6.17-rt8/kernel/futex.c
===================================================================
@@ -1303,8 +1303,10 @@
 		ret = get_user(uval, uaddr);
 		while (!ret) {
 			newval = (uval & FUTEX_OWNER_DIED) | newtid;
+			inc_preempt_count();
 			curval = futex_atomic_cmpxchg_inatomic(uaddr,
 							       uval, newval);
+			dec_preempt_count();
 			if (curval == -EFAULT)
 				ret = -EFAULT;
 			if (curval == uval)

@@ -1684,8 +1686,11 @@
 		 * thread-death.) The rest of the cleanup is done in
 		 * userspace.
 		 */
+		inc_preempt_count();
 		nval = futex_atomic_cmpxchg_inatomic(uaddr, uval,
 						     uval | FUTEX_OWNER_DIED);
+		dec_preempt_count();
+
 		if (nval == -EFAULT)
 			return -1;
--

Thanks,

Darren Hart
IBM Linux Technology Center
Realtime Linux Team