Wrap smp_processor_id in kpreempt_[dis|en]able

After surveying the code, the few places where smp_processor_id is used
were deemed to be safe to use with a preempt enabled kernel. As such, no
core logic had to be changed. These smp_processor_id call sites are simply
are wrapped in kpreempt_disable and kpreempt_enabled to prevent the
Linux kernel from emitting scary warnings.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Issue #83

include/sys/zfs_context.h

@@ -61,6 +61,7 @@
 #include <sys/fm/fs/zfs.h>
 #include <sys/sunddi.h>
 #include <sys/ctype.h>
+#include <sys/disp.h>
 #include <linux/dcache_compat.h>
 
 #else /* _KERNEL */
@@ -228,6 +229,9 @@ extern kthread_t *zk_thread_create(caddr_t stk, size_t  stksize,
 	proc_t *pp, int state, pri_t pri);
 extern void zk_thread_join(kt_did_t tid);
 
+#define	kpreempt_disable()	((void)0)
+#define	kpreempt_enable()	((void)0)
+
 #define	PS_NONE		-1
 
 #define	issig(why)	(FALSE)

module/zfs/fm.c

@@ -1413,7 +1413,13 @@ fm_ena_generate_cpu(uint64_t timestamp, processorid_t cpuid, uchar_t format)
 uint64_t
 fm_ena_generate(uint64_t timestamp, uchar_t format)
 {
-	return (fm_ena_generate_cpu(timestamp, getcpuid(), format));
+	uint64_t ena;
+
+	kpreempt_disable();
+	ena = fm_ena_generate_cpu(timestamp, getcpuid(), format);
+	kpreempt_enable();
+
+	return (ena);
 }
 
 uint64_t

module/zfs/txg.c

@@ -218,9 +218,19 @@ uint64_t
 txg_hold_open(dsl_pool_t *dp, txg_handle_t *th)
 {
 	tx_state_t *tx = &dp->dp_tx;
-	tx_cpu_t *tc = &tx->tx_cpu[CPU_SEQID];
+	tx_cpu_t *tc;
 	uint64_t txg;
 
+	/*
+	 * It appears the processor id is simply used as a "random"
+	 * number to index into the array, and there isn't any other
+	 * significance to the chosen tx_cpu. Because.. Why not use
+	 * the current cpu to index into the array?
+	 */
+	kpreempt_disable();
+	tc = &tx->tx_cpu[CPU_SEQID];
+	kpreempt_enable();
+
 	mutex_enter(&tc->tc_lock);
 
 	txg = tx->tx_open_txg;