[Gluster-devel] important change to syncop infra

Pranith Kumar Karampuri pkarampu at redhat.com
Wed Dec 11 08:51:55 UTC 2013

    We found a day-1 bug when syncop_xxx() infra is used inside a synctask with compilation optimization (CFLAGS -O2). This bug has been dormant for at least 2 years.
There are around ~400(rebalance, replace-brick, bd, self-heal-daemon, quota, fuse lock/fd migration) places where syncop is used in the code base all of which are potential candidates which can take the hit.

I sent first round of patch at http://review.gluster.com/6475 to catch regressions upstream.
These are the files that are affected by the changes I introduced to fix this:

 api/src/glfs-fops.c                             |  36 ++++++++++++++++++++++++++++++++++
 api/src/glfs-handleops.c                        |  15 ++++++++++++++
 api/src/glfs-internal.h                         |   7 +++++++
 api/src/glfs-resolve.c                          |  10 ++++++++++
 libglusterfs/src/syncop.c                       | 117 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-------------------------------------
 xlators/cluster/afr/src/afr-self-heald.c        |  45 +++++++++++++++++++++++++++++-------------
 xlators/cluster/afr/src/pump.c                  |  12 ++++++++++--
 xlators/cluster/dht/src/dht-helper.c            |  24 +++++++++++++++--------
 xlators/cluster/dht/src/dht-rebalance.c         | 168 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---------------------------------------------------------------
 xlators/cluster/dht/src/dht-selfheal.c          |   6 ++++--
 xlators/features/locks/src/posix.c              |   3 ++-
 xlators/features/qemu-block/src/bdrv-xlator.c   |  15 ++++----------
 xlators/features/qemu-block/src/qb-coroutines.c |  14 ++++++++++----
 xlators/mount/fuse/src/fuse-bridge.c            |  16 ++++++++++-----

Please review your respective component for these changes in gerrit.


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:

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,
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:
1165                        readdir_operrno = errno; <<---- errno gets expanded as *(__errno_location())
   0x00007fd149d48b60 <+496>:        callq  0x7fd149d410c0 <__errno_location at plt>
   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)
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 <__errno_location at plt><<--- 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>

We decided to 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.


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