Reorder HAVE_BIO_RW_* checks

The HAVE_BIO_RW_* #ifdef's must appear before REQ_* #ifdef's
in the bio_is_flush() and bio_is_discard() macros.  Linux 2.6.32
era kernels defined both of values and the HAVE_BIO_RW_* must be
used in this case.  This resulted in a panic in zconfig test 5.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #4951
Closes #4959

diff --git a/include/linux/blkdev_compat.h b/include/linux/blkdev_compat.h
index aa7e6b498df8d725ce6fa5e2134c027b815e8fdc..bff93ff542331a3777827dd4681d6b4abe0456ae 100644 (file)
--- a/include/linux/blkdev_compat.h
+++ b/include/linux/blkdev_compat.h
@@ -326,6 +326,10 @@ bio_set_flags_failfast(struct block_device *bdev, int *flags)
  * and the new preflush behavior introduced in Linux 4.8.  This is correct
  * in all cases but may have a performance impact for some kernels.  It
  * has the advantage of minimizing kernel specific changes in the zvol code.
+ *
+ * Note that 2.6.32 era kernels provide both BIO_RW_BARRIER and REQ_FLUSH,
+ * where BIO_RW_BARRIER is the correct interface.  Therefore, it is important
+ * that the HAVE_BIO_RW_BARRIER check occur before the REQ_FLUSH check.
  */
 static inline boolean_t
 bio_is_flush(struct bio *bio)
@@ -336,10 +340,10 @@ bio_is_flush(struct bio *bio)
        return (bio->bi_opf & REQ_PREFLUSH);
 #elif defined(REQ_PREFLUSH) && !defined(HAVE_BIO_BI_OPF)
        return (bio->bi_rw & REQ_PREFLUSH);
-#elif defined(REQ_FLUSH)
-       return (bio->bi_rw & REQ_FLUSH);
 #elif defined(HAVE_BIO_RW_BARRIER)
        return (bio->bi_rw & (1 << BIO_RW_BARRIER));
+#elif defined(REQ_FLUSH)
+       return (bio->bi_rw & REQ_FLUSH);
 #else
 #error "Allowing the build will cause flush requests to be ignored. Please "
        "file an issue report at: https://github.com/zfsonlinux/zfs/issues/new"
@@ -378,16 +382,20 @@ bio_is_fua(struct bio *bio)
  *
  * In all cases the normal I/O path is used for discards.  The only
  * difference is how the kernel tags individual I/Os as discards.
+ *
+ * Note that 2.6.32 era kernels provide both BIO_RW_DISCARD and REQ_DISCARD,
+ * where BIO_RW_DISCARD is the correct interface.  Therefore, it is important
+ * that the HAVE_BIO_RW_DISCARD check occur before the REQ_DISCARD check.
  */
 static inline boolean_t
 bio_is_discard(struct bio *bio)
 {
 #if defined(HAVE_REQ_OP_DISCARD)
        return (bio_op(bio) == REQ_OP_DISCARD);
-#elif defined(REQ_DISCARD)
-       return (bio->bi_rw & REQ_DISCARD);
 #elif defined(HAVE_BIO_RW_DISCARD)
        return (bio->bi_rw & (1 << BIO_RW_DISCARD));
+#elif defined(REQ_DISCARD)
+       return (bio->bi_rw & REQ_DISCARD);
 #else
 #error "Allowing the build will cause discard requests to become writes "
        "potentially triggering the DMU_MAX_ACCESS assertion. Please file "