Re: [Gluster-devel] Possible problem introduced by http://review.gluster.org/15573

Fri, 21 Oct 2016 06:05:47 -0700 

Hi Xavi,

On 10/21/2016 12:57 PM, Xavier Hernandez wrote:

Looking at the code, I think that the added fd_unref() should only be
called if the fop preparation fails. Otherwise the callback already
unreferences the fd.

Code flow:

* glfs_fsync_async_common() takes an fd ref and calls STACK_WIND passing
that fd.
* Just after that a ref is released.
* When glfs_io_async_cbk() is called another ref is released.

Note that if fop preparation fails, a single fd_unref() is called, but
on success two fd_unref() are called.
Sorry for the inconvenience caused. I think its patch#15573 hasn't caused the problem but has highlighted another ref leak in the code.
From the code I see that glfs_io_async_cbk() does fd_unref (glfd->fd) but not the fd passed in STACK_WIND_COOKIE() of the fop. 

If I take any fop, for eg.,
glfs_fsync_common() {

       fd = glfs_resolve_fd (glfd->fs, subvol, glfd);


}

Here in glfs_resolve_fd ()

fd_t *
__glfs_resolve_fd (struct glfs *fs, xlator_t *subvol, struct glfs_fd *glfd)
{
        fd_t *fd = NULL;

        if (glfd->fd->inode->table->xl == subvol)
                return fd_ref (glfd->fd);
>>>>>>>> Here we can see that we are taking extra ref additional to the ref already taken for glfd->fd. That means the caller of this function needs to fd_unref(fd) irrespective of subsequent fd_unref (glfd->fd). 

        fd = __glfs_migrate_fd (fs, subvol, glfd);
        if (!fd)
                return NULL;
if (subvol == fs->active_subvol) { 
                fd_unref (glfd->fd);
glfd->fd = fd_ref (fd); 
        }
>>>>>>>> I think the issue is here during graph_switch(). You have mentioned as well that the crash happens post graph_switch. Maybe here we are missing an extra ref to be taken for fd additional to glfd->fd. I need to look through __glfs_migrate_fd() to confirm that. But these are my initial thoughts. 

Please let me know your comments.

Thanks,
Soumya




Xavi

On 21/10/16 09:03, Xavier Hernandez wrote:

Hi,

I've just tried Gluster 3.8.5 with Proxmox using gfapi and I
consistently see a crash each time an attempt to connect to the volume
is made.

The backtrace of the crash shows this:

#0  pthread_spin_lock () at ../nptl/sysdeps/x86_64/pthread_spin_lock.S:24
#1  0x00007fe5345776a5 in fd_unref (fd=0x7fe523f7205c) at fd.c:553
#2  0x00007fe53482ba18 in glfs_io_async_cbk (op_ret=<optimized out>,
op_errno=0, frame=<optimized out>, cookie=0x7fe526c67040,
iovec=iovec@entry=0x0, count=count@entry=0)
    at glfs-fops.c:839
#3  0x00007fe53482beed in glfs_fsync_async_cbk (frame=<optimized out>,
cookie=<optimized out>, this=<optimized out>, op_ret=<optimized out>,
op_errno=<optimized out>,
    prebuf=<optimized out>, postbuf=0x7fe5217fe890, xdata=0x0) at
glfs-fops.c:1382
#4  0x00007fe520be2eb7 in ?? () from
/usr/lib/x86_64-linux-gnu/glusterfs/3.8.5/xlator/debug/io-stats.so
#5  0x00007fe5345d118a in default_fsync_cbk (frame=0x7fe52ceef3ac,
cookie=0x560ef95398e8, this=0x8, op_ret=0, op_errno=0, prebuf=0x1,
postbuf=0x7fe5217fe890, xdata=0x0) at defaults.c:1508
#6  0x00007fe5345d118a in default_fsync_cbk (frame=0x7fe52ceef204,
cookie=0x560ef95398e8, this=0x8, op_ret=0, op_errno=0, prebuf=0x1,
postbuf=0x7fe5217fe890, xdata=0x0) at defaults.c:1508
#7  0x00007fe525f78219 in dht_fsync_cbk (frame=0x7fe52ceef2d8,
cookie=0x560ef95398e8, this=0x0, op_ret=0, op_errno=0,
prebuf=0x7fe5217fe820, postbuf=0x7fe5217fe890, xdata=0x0)
    at dht-inode-read.c:873
#8  0x00007fe5261bbc7f in client3_3_fsync_cbk (req=0x7fe525f78030
<dht_fsync_cbk>, iov=0x7fe526c61040, count=8, myframe=0x7fe52ceef130) at
client-rpc-fops.c:975
#9  0x00007fe5343201f0 in rpc_clnt_handle_reply (clnt=0x18,
clnt@entry=0x7fe526fafac0, pollin=0x7fe526c3a1c0) at rpc-clnt.c:791
#10 0x00007fe53432056c in rpc_clnt_notify (trans=<optimized out>,
mydata=0x7fe526fafaf0, event=<optimized out>, data=0x7fe526c3a1c0) at
rpc-clnt.c:962
#11 0x00007fe53431c8a3 in rpc_transport_notify (this=<optimized out>,
event=<optimized out>, data=<optimized out>) at rpc-transport.c:541
#12 0x00007fe5283e8d96 in socket_event_poll_in (this=0x7fe526c69440) at
socket.c:2267
#13 0x00007fe5283eaf37 in socket_event_handler (fd=<optimized out>,
idx=5, data=0x7fe526c69440, poll_in=1, poll_out=0, poll_err=0) at
socket.c:2397
#14 0x00007fe5345ab3f6 in event_dispatch_epoll_handler
(event=0x7fe5217fecc0, event_pool=0x7fe526ca2040) at event-epoll.c:571
#15 event_dispatch_epoll_worker (data=0x7fe527c0f0c0) at
event-epoll.c:674
#16 0x00007fe5324140a4 in start_thread (arg=0x7fe5217ff700) at
pthread_create.c:309
#17 0x00007fe53214962d in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:111

The fd being unreferenced contains this:

(gdb) print *fd
$6 = {
  pid = 97649,
  flags = 2,
  refcount = 0,
  inode_list = {
    next = 0x7fe523f7206c,
    prev = 0x7fe523f7206c
  },
  inode = 0x0,
  lock = {
    spinlock = 1,
    mutex = {
      __data = {
        __lock = 1,
        __count = 0,
        __owner = 0,
        __nusers = 0,
        __kind = 0,
        __spins = 0,
        __elision = 0,
        __list = {
          __prev = 0x0,
          __next = 0x0
        }
      },
      __size = "\001", '\000' <repeats 38 times>,
      __align = 1
    }
  },
  _ctx = 0x7fe52ec31c40,
  xl_count = 11,
  lk_ctx = 0x7fe526c126a0,
  anonymous = _gf_false
}

fd->inode is NULL, explaining the cause of the crash. We also see that
fd->refcount is already 0. So I'm wondering if this couldn't be an extra
fd_unref() introduced by that patch.

The crash seems to happen immediately after a graph switch.

Xavi
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