[lustre-devel] [PATCH 02/25] lustre: llite: No locked parallel DIO

James Simmons jsimmons at infradead.org
Mon Aug 2 12:50:22 PDT 2021

From: Patrick Farrell <pfarrell at whamcloud.com>

If we are doing locked DIO, the OSC & LDLM locks are
released at the end of cl_io_loop, ie, before we wait for
parallel DIO at the llite layer.

This is problematic because the locks are released before
i/o done using them is complete; this can lead to data
inconsistencies.  (And at least one LBUG, see LU-14805.)

The easiest solution for now is only do parallel DIO when
working lockless (which is the default; DIO only switches
to locked to manage conflicts with buffered i/o).

This problem & fix apply to AIO as well as parallel DIO.

WC-bug-id: https://jira.whamcloud.com/browse/LU-14805
Lustre-commit: 0f8db7e06abbc341 ("LU-14805 llite: No locked parallel DIO")
Signed-off-by: Patrick Farrell <pfarrell at whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44131
Reviewed-by: Wang Shilong <wshilong at whamcloud.com>
Reviewed-by: Andreas Dilger <adilger at whamcloud.com>
Reviewed-by: Oleg Drokin <green at whamcloud.com>
Signed-off-by: James Simmons <jsimmons at infradead.org>
 fs/lustre/llite/rw26.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/lustre/llite/rw26.c b/fs/lustre/llite/rw26.c
index ba9c070..0d72c3e 100644
--- a/fs/lustre/llite/rw26.c
+++ b/fs/lustre/llite/rw26.c
@@ -410,10 +410,19 @@ static ssize_t ll_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 		vio->u.readwrite.vui_read += tot_bytes;
-	/* If async dio submission is not allowed, we must wait here. */
-	if (is_sync_kiocb(iocb) && !io->ci_parallel_dio) {
+	/* We cannot do async submission - for AIO or regular DIO - unless
+	 * lockless because it causes us to release the lock early.
+	 *
+	 * There are also several circumstances in which we must disable
+	 * parallel DIO, so we check if it is enabled.
+	 *
+	 * The check for "is_sync_kiocb" excludes AIO, which does not need to
+	 * be disabled in these situations.
+	 */
+	if (io->ci_dio_lock || (is_sync_kiocb(iocb) && !io->ci_parallel_dio)) {
 		ssize_t rc2;
+		/* Wait here rather than doing async submission */
 		rc2 = cl_sync_io_wait_recycle(env, &aio->cda_sync, 0, 0);
 		if (result == 0 && rc2)
 			result = rc2;

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