Eliminate the PC_CURTHREAD symbol and load the current thread's

thread structure pointer atomically from r13 (the pcpu pointer)
for the current CPU/core.
Add a CTASSERT in machdep.c to make sure that pc_curthread is in
fact the first field in struct pcpu.

The only non-atomic operations left were those related to process-
space operations, such as casuword, subyte, suword16, fubyte,
fuword16, copyin, copyout and their variations.

The casuword function has been re-structured more complete than
the others. This way we have an example of a better bundling
without introducing a lot of risk when we get it wrong. The
other functions can be rebundled in separate commits and with
the appropriate testing.

1 files changed, 16 insertions, 0 deletions

diff --git a/sys/ia64/ia64/machdep.c b/sys/ia64/ia64/machdep.c
index fbc35d1..98bcf44 100644
--- a/sys/ia64/ia64/machdep.c
+++ b/sys/ia64/ia64/machdep.c
@@ -98,6 +98,22 @@ __FBSDID("$FreeBSD$");
 #include <machine/unwind.h>
 #include <machine/vmparam.h>
 
+/*
+ * For atomicity reasons, we demand that pc_curthread is the first
+ * field in the struct pcpu. It allows us to read the pointer with
+ * a single atomic instruction:
+ *	ld8 %curthread = [r13]
+ * Otherwise we would first have to calculate the load address and
+ * store the result in a temporary register and that for the load:
+ *	add %temp = %offsetof(struct pcpu), r13
+ *	ld8 %curthread = [%temp]
+ * A context switch inbetween the add and the ld8 could have the
+ * thread migrate to a different core. In that case,  %curthread
+ * would be the thread running on the original core and not actually
+ * the current thread.
+ */
+CTASSERT(offsetof(struct pcpu, pc_curthread) == 0);
+
 static SYSCTL_NODE(_hw, OID_AUTO, freq, CTLFLAG_RD, 0, "");
 static SYSCTL_NODE(_machdep, OID_AUTO, cpu, CTLFLAG_RD, 0, "");