[PATCH printk-rework 10/12] hv: synchronize kmsg_dumper

[John Ogness]

In preparation for removing printk's @logbuf_lock, dumpers that have
assumed to be protected against parallel calls must provide their own
synchronization. Add a locally static spinlock to synchronize the
kmsg_dump call.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 drivers/hv/vmbus_drv.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index fbeddef90941..08db95e1a139 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1362,23 +1362,27 @@ static void hv_kmsg_dump(struct kmsg_dumper *dumper,
 			 enum kmsg_dump_reason reason,
 			 struct kmsg_dumper_iter *iter)
 {
+	static DEFINE_SPINLOCK(lock);
 	size_t bytes_written;
 	phys_addr_t panic_pa;
+	unsigned long flags;
 
 	/* We are only interested in panics. */
 	if ((reason != KMSG_DUMP_PANIC) || (!sysctl_record_panic_msg))
 		return;
 
+	if (!spin_trylock_irqsave(&lock, flags))
+		return;
+
 	panic_pa = virt_to_phys(hv_panic_page);
 
-	/*
-	 * Write dump contents to the page. No need to synchronize; panic should
-	 * be single-threaded.
-	 */
+	/* Write dump contents to the page. */
 	kmsg_dump_get_buffer(iter, false, hv_panic_page, HV_HYP_PAGE_SIZE,
 			     &bytes_written);
 	if (bytes_written)
 		hyperv_report_panic_msg(panic_pa, bytes_written);
+
+	spin_unlock_irqrestore(&lock, flags);
 }
 
 static struct kmsg_dumper hv_kmsg_dumper = {
-- 
2.20.1