generic-ipi: fix the smp_mb() placement

smp_mb() is needed (to make the memory operations visible globally) before
sending the ipi on the sender and the receiver (on Alpha atleast) needs
smp_read_barrier_depends() in the handler before reading the call_single_queue
list in a lock-free fashion.

On x86, x2apic mode register accesses for sending IPI's don't have serializing
semantics. So the need for smp_mb() before sending the IPI becomes more
critical in x2apic mode.

Remove the unnecessary smp_mb() in csd_flag_wait(), as the presence of that
smp_mb() doesn't mean anything on the sender, when the ipi receiver is not
doing any thing special (like memory fence) after clearing the CSD_FLAG_WAIT.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/kernel/smp.c b/kernel/smp.c
index f362a85537777619d1ef22602042fba351a75e7d..75c8dde58c55fcdd2f0ae71f75ceb05213e0985e 100644 (file)
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -51,10 +51,6 @@ static void csd_flag_wait(struct call_single_data *data)
 {
        /* Wait for response */
        do {
-               /*
-                * We need to see the flags store in the IPI handler
-                */
-               smp_mb();
                if (!(data->flags & CSD_FLAG_WAIT))
                        break;
                cpu_relax();
@@ -76,6 +72,11 @@ static void generic_exec_single(int cpu, struct call_single_data *data)
        list_add_tail(&data->list, &dst->list);
        spin_unlock_irqrestore(&dst->lock, flags);
 
+       /*
+        * Make the list addition visible before sending the ipi.
+        */
+       smp_mb();
+
        if (ipi)
                arch_send_call_function_single_ipi(cpu);
 
@@ -157,7 +158,7 @@ void generic_smp_call_function_single_interrupt(void)
         * Need to see other stores to list head for checking whether
         * list is empty without holding q->lock
         */
-       smp_mb();
+       smp_read_barrier_depends();
        while (!list_empty(&q->list)) {
                unsigned int data_flags;
 
@@ -191,7 +192,7 @@ void generic_smp_call_function_single_interrupt(void)
                /*
                 * See comment on outer loop
                 */
-               smp_mb();
+               smp_read_barrier_depends();
        }
 }
 
@@ -370,6 +371,11 @@ int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
        list_add_tail_rcu(&data->csd.list, &call_function_queue);
        spin_unlock_irqrestore(&call_function_lock, flags);
 
+       /*
+        * Make the list addition visible before sending the ipi.
+        */
+       smp_mb();
+
        /* Send a message to all CPUs in the map */
        arch_send_call_function_ipi(mask);