xfs: avoid unnecessary waits in xfs_log_force_lsn()

Before waiting on a iclog in xfs_log_force_lsn(), we don't check to
see if the iclog has already been completed and the contents on
stable storage. We check for completed iclogs in xfs_log_force(), so
we should do the same thing for xfs_log_force_lsn().

This fixed some random up-to-30s pauses seen in unmounting
filesystems in some tests. A log force ends up waiting on completed
iclog, and that doesn't then get flushed (and hence the log force
get completed) until the background log worker issues a log force
that flushes the iclog in question. Then the unmount unblocks and
continues.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 160b8bb7ee604bd2088d79646c3ee2441b6f0f2d..1c328efdca66fabdc12d21101c15b3e0f8115983 100644 (file)
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -3143,6 +3143,35 @@ xlog_state_switch_iclogs(
        log->l_iclog = iclog->ic_next;
 }
 
+/*
+ * Force the iclog to disk and check if the iclog has been completed before
+ * xlog_force_iclog() returns. This can happen on synchronous (e.g.
+ * pmem) or fast async storage because we drop the icloglock to issue the IO.
+ * If completion has already occurred, tell the caller so that it can avoid an
+ * unnecessary wait on the iclog.
+ */
+static int
+xlog_force_and_check_iclog(
+       struct xlog_in_core     *iclog,
+       bool                    *completed)
+{
+       xfs_lsn_t               lsn = be64_to_cpu(iclog->ic_header.h_lsn);
+       int                     error;
+
+       *completed = false;
+       error = xlog_force_iclog(iclog);
+       if (error)
+               return error;
+
+       /*
+        * If the iclog has already been completed and reused the header LSN
+        * will have been rewritten by completion
+        */
+       if (be64_to_cpu(iclog->ic_header.h_lsn) != lsn)
+               *completed = true;
+       return 0;
+}
+
 /*
  * Write out all data in the in-core log as of this exact moment in time.
  *
@@ -3177,7 +3206,6 @@ xfs_log_force(
 {
        struct xlog             *log = mp->m_log;
        struct xlog_in_core     *iclog;
-       xfs_lsn_t               lsn;
 
        XFS_STATS_INC(mp, xs_log_force);
        trace_xfs_log_force(mp, 0, _RET_IP_);
@@ -3206,11 +3234,12 @@ xfs_log_force(
        } else if (iclog->ic_state == XLOG_STATE_ACTIVE) {
                if (atomic_read(&iclog->ic_refcnt) == 0) {
                        /* We have exclusive access to this iclog. */
-                       lsn = be64_to_cpu(iclog->ic_header.h_lsn);
-                       if (xlog_force_iclog(iclog))
+                       bool    completed;
+
+                       if (xlog_force_and_check_iclog(iclog, &completed))
                                goto out_error;
 
-                       if (be64_to_cpu(iclog->ic_header.h_lsn) != lsn)
+                       if (completed)
                                goto out_unlock;
                } else {
                        /*
@@ -3250,6 +3279,7 @@ xlog_force_lsn(
        bool                    already_slept)
 {
        struct xlog_in_core     *iclog;
+       bool                    completed;
 
        spin_lock(&log->l_icloglock);
        iclog = log->l_iclog;
@@ -3287,10 +3317,12 @@ xlog_force_lsn(
                                        &log->l_icloglock);
                        return -EAGAIN;
                }
-               if (xlog_force_iclog(iclog))
+               if (xlog_force_and_check_iclog(iclog, &completed))
                        goto out_error;
                if (log_flushed)
                        *log_flushed = 1;
+               if (completed)
+                       goto out_unlock;
                break;
        case XLOG_STATE_WANT_SYNC:
                /*