MFC r322239: MFV r322238: 7915 checks in l2arc_evict could use some cleaning up

illumos/illumos-gate@267ae6c3a88d2fc39276af66caafa978b0935b82
https://github.com/illumos/illumos-gate/commit/267ae6c3a88d2fc39276af66caafa978b0935b82

https://www.illumos.org/issues/7915
  l2arc_evict() is strictly serialized with respect to
  l2arc_write_buffers() and l2arc_write_done().  Normally, l2arc_evict()
  and l2arc_write_buffers() are called from the same thread, so they can
  not be concurrent.  Also, l2arc_write_buffers() uses zio_wait() on the
  parent zio of all cache zio-s.  That ensures that l2arc_write_done()
  is completed before l2arc_write_buffers() returns.  Finally, if a
  cache device is removed, then l2arc_evict() is called under SCL_ALL in
  the exclusive mode.  That ensures that it can not be concurrent with
  the normal L2ARC accesses to the device (including writing and
  evicting buffers).  Given the above, some checks and actions in
  l2arc_evict() do not make sense.  For instance, it must never
  encounter the write head header let alone remove it from the buffer
  list.

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Author: Andriy Gapon <avg@FreeBSD.org>

1 files changed, 9 insertions, 15 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
index daa3f26..2909032 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
@@ -7314,18 +7314,16 @@ top:
 			goto top;
 		}
 
-		if (HDR_L2_WRITE_HEAD(hdr)) {
-			/*
-			 * We hit a write head node.  Leave it for
-			 * l2arc_write_done().
-			 */
-			list_remove(buflist, hdr);
-			mutex_exit(hash_lock);
-			continue;
-		}
+		/*
+		 * A header can't be on this list if it doesn't have L2 header.
+		 */
+		ASSERT(HDR_HAS_L2HDR(hdr));
 
-		if (!all && HDR_HAS_L2HDR(hdr) &&
-		    (hdr->b_l2hdr.b_daddr >= taddr ||
+		/* Ensure this header has finished being written. */
+		ASSERT(!HDR_L2_WRITING(hdr));
+		ASSERT(!HDR_L2_WRITE_HEAD(hdr));
+
+		if (!all && (hdr->b_l2hdr.b_daddr >= taddr ||
 		    hdr->b_l2hdr.b_daddr < dev->l2ad_hand)) {
 			/*
 			 * We've evicted to the target address,
@@ -7335,7 +7333,6 @@ top:
 			break;
 		}
 
-		ASSERT(HDR_HAS_L2HDR(hdr));
 		if (!HDR_HAS_L1HDR(hdr)) {
 			ASSERT(!HDR_L2_READING(hdr));
 			/*
@@ -7358,9 +7355,6 @@ top:
 				arc_hdr_set_flags(hdr, ARC_FLAG_L2_EVICTED);
 			}
 
-			/* Ensure this header has finished being written */
-			ASSERT(!HDR_L2_WRITING(hdr));
-
 			arc_hdr_l2hdr_destroy(hdr);
 		}
 		mutex_exit(hash_lock);