syncop: Change return value of syncop

Problem:
We found a day-1 bug when syncop_xxx() infra is used inside a synctask with
compilation optimization (CFLAGS -O2).

Detailed explanation of the Root cause:
We found the bug in 'gf_defrag_migrate_data' in rebalance operation:

Lets look at interesting parts of the function:

int
gf_defrag_migrate_data (xlator_t *this, gf_defrag_info_t *defrag, loc_t *loc,
                        dict_t *migrate_data)
{
.....
code section - [ Loop ]
        while ((ret = syncop_readdirp (this, fd, 131072, offset, NULL,
                                       &entries)) != 0) {
.....
code section - [ ERRNO-1 ] (errno of readdirp is stored in readdir_operrno by a
thread)
                /* Need to keep track of ENOENT errno, that means, there is no
                   need to send more readdirp() */
                readdir_operrno = errno;
.....
code section - [ SYNCOP-1 ] (syncop_getxattr is called by a thread)
                        ret = syncop_getxattr (this, &entry_loc, &dict,
                                               GF_XATTR_LINKINFO_KEY);
code section - [ ERRNO-2]   (checking for failures of syncop_getxattr(). This
may not always be executed in same thread which executed [SYNCOP-1])
                        if (ret < 0) {
                                if (errno != ENODATA) {
                                        loglevel = GF_LOG_ERROR;
                                        defrag->total_failures += 1;
.....
}

the function above could be executed by thread(t1) till [SYNCOP-1] and code
from [ERRNO-2] can be executed by a different thread(t2) because of the way
syncop-infra schedules the tasks.

when the code is compiled with -O2 optimization this is the assembly code that
is generated:
 [ERRNO-1]
1165                        readdir_operrno = errno; <<---- errno gets expanded
as *(__errno_location())
   0x00007fd149d48b60 <+496>:        callq  0x7fd149d410c0 <address@hidden>
   0x00007fd149d48b72 <+514>:        mov    %rax,0x50(%rsp) <<------ Address
returned by __errno_location() is stored in a special location in stack for
later use.
   0x00007fd149d48b77 <+519>:        mov    (%rax),%eax
   0x00007fd149d48b79 <+521>:        mov    %eax,0x78(%rsp)
....
 [ERRNO-2]
1281                                        if (errno != ENODATA) {
   0x00007fd149d492ae <+2366>:        mov    0x50(%rsp),%rax <<-----  Because
it already stored the address returned by __errno_location(), it just
dereferences the address to get the errno value. BUT THIS CODE NEED NOT BE
EXECUTED BY SAME THREAD!!!
   0x00007fd149d492b3 <+2371>:        mov    $0x9,%ebp
   0x00007fd149d492b8 <+2376>:        mov    (%rax),%edi
   0x00007fd149d492ba <+2378>:        cmp    $0x3d,%edi

The problem is that __errno_location() value of t1 and t2 are different. So
[ERRNO-2] ends up reading errno of t1 instead of errno of t2 even though t2 is
executing [ERRNO-2] code section.

When code is compiled without any optimization for [ERRNO-2]:
1281                                        if (errno != ENODATA) {
   0x00007fd58e7a326f <+2237>:        callq  0x7fd58e797300
<address@hidden><<--- As it is calling __errno_location() again it gets the
location from t2 so it works as intended.
   0x00007fd58e7a3274 <+2242>:        mov    (%rax),%eax
   0x00007fd58e7a3276 <+2244>:        cmp    $0x3d,%eax
   0x00007fd58e7a3279 <+2247>:        je     0x7fd58e7a32a1
<gf_defrag_migrate_data+2287>

Fix:
Make syncop_xxx() return (-errno) value as the return value in
case of errors and all the functions which make syncop_xxx() will need to use
(-ret) to figure out the reason for failure in case of syncop_xxx() failures.

Change-Id: I314d20dabe55d3e62ff66f3b4adb1cac2eaebb57
BUG: 1040356
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/6475
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>

2 files changed, 14 insertions, 16 deletions

diff --git a/xlators/features/qemu-block/src/bdrv-xlator.c b/xlators/features/qemu-block/src/bdrv-xlator.c
index 106c5977535..aaf028cfea5 100644
--- a/xlators/features/qemu-block/src/bdrv-xlator.c
+++ b/xlators/features/qemu-block/src/bdrv-xlator.c
@@ -109,7 +109,7 @@ qemu_gluster_open (BlockDriverState *bs, QDict *options, int bdrv_flags)
 				    NULL);
 		if (ret) {
 			loc_wipe(&loc);
-			return -errno;
+			return ret;
 		}
 
 		s->inode = inode_ref(loc.inode);
@@ -153,7 +153,7 @@ qemu_gluster_create (const char *filename, QEMUOptionParameter *options)
 	ret = syncop_fstat (FIRST_CHILD(THIS), fd, &stat);
 	if (ret) {
 		fd_unref (fd);
-		return -errno;
+		return ret;
 	}
 
 	if (stat.ia_size) {
@@ -166,7 +166,7 @@ qemu_gluster_create (const char *filename, QEMUOptionParameter *options)
 		ret = syncop_ftruncate (FIRST_CHILD(THIS), fd, total_size);
 		if (ret) {
 			fd_unref (fd);
-			return -errno;
+			return ret;
 		}
 	}
 
@@ -196,10 +196,8 @@ qemu_gluster_co_readv (BlockDriverState *bs, int64_t sector_num, int nb_sectors,
 
 	ret = syncop_readv (FIRST_CHILD(THIS), fd, size, offset, 0,
 			    &iov, &count, &iobref);
-	if (ret < 0) {
-		ret = -errno;
+	if (ret < 0)
 		goto out;
-	}
 
 	iov_copy (qiov->iov, qiov->niov, iov, count); /* *choke!* */
 
@@ -249,8 +247,6 @@ qemu_gluster_co_writev (BlockDriverState *bs, int64_t sector_num, int nb_sectors
 	iov.iov_len = size;
 
 	ret = syncop_writev (FIRST_CHILD(THIS), fd, &iov, 1, offset, iobref, 0);
-	if (ret < 0)
-		ret = -errno;
 
 out:
 	if (iobuf)
@@ -306,9 +302,6 @@ qemu_gluster_truncate (BlockDriverState *bs, int64_t offset)
 
 	fd_unref (fd);
 
-	if (ret < 0)
-		return ret;
-
 	return ret;
 }
 
diff --git a/xlators/features/qemu-block/src/qb-coroutines.c b/xlators/features/qemu-block/src/qb-coroutines.c
index 7c52adb21ed..974312f1268 100644
--- a/xlators/features/qemu-block/src/qb-coroutines.c
+++ b/xlators/features/qemu-block/src/qb-coroutines.c
@@ -86,7 +86,7 @@ qb_format_and_resume (void *opaque)
 		GF_FREE(qb_inode->backing_fname);
 		if (ret) {
 			loc_wipe(&loc);
-			ret = errno;
+			ret = -ret;
 			goto err;
 		}
 
@@ -150,11 +150,10 @@ qb_format_and_resume (void *opaque)
 
 	ret = syncop_fsetxattr (FIRST_CHILD(THIS), fd, xattr, 0);
 	if (ret) {
-		ret = errno;
 		gf_log (frame->this->name, GF_LOG_ERROR,
 			"failed to setxattr for %s",
 			uuid_utoa (inode->gfid));
-		QB_STUB_UNWIND (stub, -1, ret);
+		QB_STUB_UNWIND (stub, -1, -ret);
 		fd_unref (fd);
 		dict_unref (xattr);
 		return 0;
@@ -476,7 +475,10 @@ qb_co_truncate (void *opaque)
 		}
 	}
 
-	syncop_fstat (FIRST_CHILD(this), local->fd, &stub->args_cbk.prestat);
+	ret = syncop_fstat (FIRST_CHILD(this), local->fd,
+                            &stub->args_cbk.prestat);
+        if (ret < 0)
+                goto out;
 	stub->args_cbk.prestat.ia_size = qb_inode->size;
 
 	ret = bdrv_truncate (qb_inode->bs, stub->args.offset);
@@ -487,7 +489,10 @@ qb_co_truncate (void *opaque)
 
 	qb_inode->size = offset;
 
-	syncop_fstat (FIRST_CHILD(this), local->fd, &stub->args_cbk.poststat);
+	ret = syncop_fstat (FIRST_CHILD(this), local->fd,
+                            &stub->args_cbk.poststat);
+        if (ret < 0)
+                goto out;
 	stub->args_cbk.poststat.ia_size = qb_inode->size;
 
 	qb_update_size_xattr (this, local->fd, qb_inode->fmt, qb_inode->size);