On 05/17/2011 05:16 AM, John Baldwin wrote:
...
Index: kern/kern_switch.c
===================================================================
--- kern/kern_switch.c (revision 221536)
+++ kern/kern_switch.c (working copy)
@@ -192,15 +192,22 @@
critical_exit(void)
{
struct thread *td;
- int flags;
+ int flags, owepreempt;
td = curthread;
KASSERT(td->td_critnest != 0,
("critical_exit: td_critnest == 0"));
if (td->td_critnest == 1) {
+ owepreempt = td->td_owepreempt;
+ td->td_owepreempt = 0;
+ /*
+ * XXX: Should move compiler_memory_barrier() from
+ * rmlock to a header.
+ */
XXX: If we get an interrupt at this point and td_owepreempt was zero,
the new interrupt will re-set it, because td_critnest is still non-zero.
So we still end up with a thread that is leaking an owepreempt *and*
lose a preemption.
We really need an atomic_readandclear() which gives us a local copy of
td_owepreempt *and* clears critnest in the same operation. Sadly, that
is rather expensive. It is possible to implement with a flag for
owepreempt, but that means that all writes to critnest must then be
atomic. Either because we know we have interrupts disabled (i.e.
setting owepreempt can be a RMW), or with a proper atomic_add/set/...
I'm not sure what the performance impact of this will be. One would
hope that atomic_add without a memory barrier isn't much more expensive
than a compiler generated read-modify-write, tho. Especially, since
this cacheline should be local and exclusive to us, anyway.
+ __asm __volatile("":::"memory");
td->td_critnest = 0;
- if (td->td_owepreempt) {
+ if (owepreempt) {
td->td_critnest = 1;
thread_lock(td);
td->td_critnest--;
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