x86: Reduce amount of time the MCA lock is held while emitting records

The MCA spin lock is acquired in the hardware interrupt context to
record MCA messages. It is also acquired by a task handler to emit
those messages.

Reduce the amount of time the task handler holds the lock to reduce
the maximum amount of time the hardware interrupt handler may need to
spin on the lock.

Reviewed by:	glebius, markj
MFC after:	2 weeks
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D52938

1 files changed, 15 insertions, 3 deletions

diff --git a/sys/x86/x86/mca.c b/sys/x86/x86/mca.c
index e43c88b3a27b..7ee22895e0a7 100644
--- a/sys/x86/x86/mca.c
+++ b/sys/x86/x86/mca.c
@@ -796,9 +796,9 @@ mca_record_entry(enum scan_mode mode, const struct mca_record *record)
 		mtx_lock_spin(&mca_lock);
 		rec = STAILQ_FIRST(&mca_freelist);
 		if (rec == NULL) {
+			mtx_unlock_spin(&mca_lock);
 			printf("MCA: Unable to allocate space for an event.\n");
 			mca_log(record);
-			mtx_unlock_spin(&mca_lock);
 			return;
 		}
 		STAILQ_REMOVE_HEAD(&mca_freelist, link);
@@ -1017,6 +1017,7 @@ static void
 mca_process_records(enum scan_mode mode)
 {
 	struct mca_internal *mca;
+	STAILQ_HEAD(, mca_internal) tmplist;
 
 	/*
 	 * If in an interrupt context, defer the post-scan activities to a
@@ -1028,10 +1029,21 @@ mca_process_records(enum scan_mode mode)
 		return;
 	}
 
+	/*
+	 * Copy the pending list to the stack so we can drop the spin lock
+	 * while we are emitting logs.
+	 */
+	STAILQ_INIT(&tmplist);
 	mtx_lock_spin(&mca_lock);
-	while ((mca = STAILQ_FIRST(&mca_pending)) != NULL) {
-		STAILQ_REMOVE_HEAD(&mca_pending, link);
+	STAILQ_SWAP(&mca_pending, &tmplist, mca_internal);
+	mtx_unlock_spin(&mca_lock);
+
+	STAILQ_FOREACH(mca, &tmplist, link)
 		mca_log(&mca->rec);
+
+	mtx_lock_spin(&mca_lock);
+	while ((mca = STAILQ_FIRST(&tmplist)) != NULL) {
+		STAILQ_REMOVE_HEAD(&tmplist, link);
 		mca_store_record(mca);
 	}
 	mtx_unlock_spin(&mca_lock);