Author: andrew
Date: Mon Sep 7 14:01:18 2015
New Revision: 287536
URL: https://svnweb.freebsd.org/changeset/base/287536
Log:
Use load-acquire semantics while waiting for td_lock to be released. The
store should have release semantics and will have due to the dsb above it
so add a comment to explain this. [1]
While here update the code to not reload the current thread, it's already
in a register, we just need to not trash it.
Suggested by: kib [1]
Sponsored by: ABT Systems Ltd
Modified:
head/sys/arm64/arm64/swtch.S
Modified: head/sys/arm64/arm64/swtch.S
==============================================================================
--- head/sys/arm64/arm64/swtch.S Mon Sep 7 14:00:38 2015
(r287535)
+++ head/sys/arm64/arm64/swtch.S Mon Sep 7 14:01:18 2015
(r287536)
@@ -160,17 +160,18 @@ ENTRY(cpu_switch)
dsb sy
isb
- /* Release the old thread */
+ /*
+ * Release the old thread. This doesn't need to be a store-release
+ * as the above dsb instruction will provide release semantics.
+ */
str x2, [x0, #TD_LOCK]
#if defined(SCHED_ULE) && defined(SMP)
/* Read the value in blocked_lock */
ldr x0, =_C_LABEL(blocked_lock)
- ldr x1, [x0]
- /* Load curthread */
- ldr x2, [x18, #PC_CURTHREAD]
+ ldr x2, [x0]
1:
- ldr x3, [x2, #TD_LOCK]
- cmp x3, x1
+ ldar x3, [x1, #TD_LOCK]
+ cmp x3, x2
b.eq 1b
#endif
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