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>

1 files changed, 10 insertions, 0 deletions

diff --git a/api/src/glfs-resolve.c b/api/src/glfs-resolve.c
index 4ca2eb6fcf4..c88330aa48e 100644
--- a/api/src/glfs-resolve.c
+++ b/api/src/glfs-resolve.c
@@ -48,6 +48,7 @@ glfs_first_lookup_safe (xlator_t *subvol)
 	loc.name = "";
 
 	ret = syncop_lookup (subvol, &loc, 0, 0, 0, 0);
+        DECODE_SYNCOP_ERR (ret);
 
 	gf_log (subvol->name, GF_LOG_DEBUG, "first lookup complete %d", ret);
 
@@ -98,6 +99,7 @@ glfs_refresh_inode_safe (xlator_t *subvol, inode_t *oldinode)
 		return NULL;
 
 	ret = syncop_lookup (subvol, &loc, 0, &iatt, 0, 0);
+        DECODE_SYNCOP_ERR (ret);
 
 	if (ret) {
 		gf_log (subvol->name, GF_LOG_WARNING,
@@ -181,6 +183,7 @@ glfs_resolve_symlink (struct glfs *fs, xlator_t *subvol, inode_t *inode,
 	loc.path = rpath;
 
 	ret = syncop_readlink (subvol, &loc, &path, 4096);
+        DECODE_SYNCOP_ERR (ret);
 
 	if (ret < 0)
 		goto out;
@@ -210,6 +213,7 @@ glfs_resolve_base (struct glfs *fs, xlator_t *subvol, inode_t *inode,
 		goto out;
 
 	ret = syncop_lookup (subvol, &loc, NULL, iatt, NULL, NULL);
+        DECODE_SYNCOP_ERR (ret);
 out:
 	loc_wipe (&loc);
 
@@ -268,6 +272,7 @@ glfs_resolve_component (struct glfs *fs, xlator_t *subvol, inode_t *parent,
 	}
 
 	ret = syncop_lookup (subvol, &loc, NULL, &ciatt, NULL, NULL);
+        DECODE_SYNCOP_ERR (ret);
 	if (ret && reval) {
 		inode_unref (loc.inode);
 		loc.inode = inode_new (parent->table);
@@ -292,6 +297,7 @@ glfs_resolve_component (struct glfs *fs, xlator_t *subvol, inode_t *parent,
 
 		ret = syncop_lookup (subvol, &loc, xattr_req, &ciatt,
 				     NULL, NULL);
+                DECODE_SYNCOP_ERR (ret);
 	}
 	if (ret)
 		goto out;
@@ -515,6 +521,7 @@ glfs_migrate_fd_locks_safe (struct glfs *fs, xlator_t *oldsubvol, fd_t *oldfd,
 
 	ret = syncop_fgetxattr (oldsubvol, oldfd, &lockinfo,
 				GF_XATTR_LOCKINFO_KEY);
+        DECODE_SYNCOP_ERR (ret);
 	if (ret < 0) {
 		gf_log (fs->volname, GF_LOG_WARNING,
 			"fgetxattr (%s) failed (%s) on graph %s (%d)",
@@ -533,6 +540,7 @@ glfs_migrate_fd_locks_safe (struct glfs *fs, xlator_t *oldsubvol, fd_t *oldfd,
 	}
 
 	ret = syncop_fsetxattr (newsubvol, newfd, lockinfo, 0);
+        DECODE_SYNCOP_ERR (ret);
 	if (ret < 0) {
 		gf_log (fs->volname, GF_LOG_WARNING,
 			"fsetxattr (%s) failed (%s) on graph %s (%d)",
@@ -568,6 +576,7 @@ glfs_migrate_fd_safe (struct glfs *fs, xlator_t *newsubvol, fd_t *oldfd)
 
 	if (!oldsubvol->switched) {
 		ret = syncop_fsync (oldsubvol, oldfd, 0);
+                DECODE_SYNCOP_ERR (ret);
 		if (ret) {
 			gf_log (fs->volname, GF_LOG_WARNING,
 				"fsync() failed (%s) on %s graph %s (%d)",
@@ -614,6 +623,7 @@ glfs_migrate_fd_safe (struct glfs *fs, xlator_t *newsubvol, fd_t *oldfd)
 		ret = syncop_open (newsubvol, &loc,
 				   oldfd->flags & ~(O_TRUNC|O_EXCL|O_CREAT),
 				   newfd);
+        DECODE_SYNCOP_ERR (ret);
 	loc_wipe (&loc);
 
 	if (ret) {