[Devel] [PATCH RHEL7 COMMIT] ms/errseq: Always report a writeback error once

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-862.3.2.vz7.61.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-862.3.2.vz7.61.3
------>
commit 85d8f115e1059c966809c46a778af6fefcaa3f51
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Mon Jun 18 12:02:04 2018 +0300

    ms/errseq: Always report a writeback error once
    
    The errseq_t infrastructure assumes that errors which occurred before
    the file descriptor was opened are of no interest to the application.
    This turns out to be a regression for some applications, notably Postgres.
    
    Before errseq_t, a writeback error would be reported exactly once (as
    long as the inode remained in memory), so Postgres could open a file,
    call fsync() and find out whether there had been a writeback error on
    that file from another process.
    
    This patch changes the errseq infrastructure to report errors to all
    file descriptors which are opened after the error occurred, but before
    it was reported to any file descriptor.  This restores the user-visible
    behaviour.
    
    Cc: stable at vger.kernel.org
    Fixes: 5660e13d2fd6 ("fs: new infrastructure for writeback error handling and reporting")
    Signed-off-by: Matthew Wilcox <mawilcox at microsoft.com>
    Reviewed-by: Jeff Layton <jlayton at kernel.org>
    Signed-off-by: Jeff Layton <jlayton at redhat.com>
    
    https://pmc.acronis.com/browse/VSTOR-10912
    (cherry picked from commit b4678df184b314a2bd47d2329feca2c2534aa12b)
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 lib/errseq.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/lib/errseq.c b/lib/errseq.c
index 7b900c2a277a..c0fc52df4d1d 100644
--- a/lib/errseq.c
+++ b/lib/errseq.c
@@ -110,25 +110,22 @@ EXPORT_SYMBOL(errseq_set);
  * errseq_sample - grab current errseq_t value
  * @eseq: pointer to errseq_t to be sampled
  *
- * This function allows callers to sample an errseq_t value, marking it as
- * "seen" if required.
+ * This function allows callers to initialise their errseq_t variable.
+ * If the error has been "seen", new callers will not see an old error.
+ * If there is an unseen error in @eseq, the caller of this function will
+ * see it the next time it checks for an error.
+ *
+ * Context: Any context.
+ * Return: The current errseq value.
  */
 errseq_t errseq_sample(errseq_t *eseq)
 {
 	errseq_t old = READ_ONCE(*eseq);
-	errseq_t new = old;
 
-	/*
-	 * For the common case of no errors ever having been set, we can skip
-	 * marking the SEEN bit. Once an error has been set, the value will
-	 * never go back to zero.
-	 */
-	if (old != 0) {
-		new |= ERRSEQ_SEEN;
-		if (old != new)
-			cmpxchg(eseq, old, new);
-	}
-	return new;
+	/* If nobody has seen this error yet, then we can be the first. */
+	if (!(old & ERRSEQ_SEEN))
+		old = 0;
+	return old;
 }
 EXPORT_SYMBOL(errseq_sample);