[Gluster-devel] Regarding doing away with refkeeper in locks xlator

[Anand Avati]

On 6/3/14, 11:32 PM, Pranith Kumar Karampuri wrote:
>
> On 06/04/2014 11:37 AM, Krutika Dhananjay wrote:
>> Hi,
>>
>> Recently there was a crash in locks translator (BZ 1103347, BZ
>> 1097102) with the following backtrace:
>> (gdb) bt
>> #0  uuid_unpack (in=0x8 <Address 0x8 out of bounds>,
>> uu=0x7fffea6c6a60) at ../../contrib/uuid/unpack.c:44
>> #1  0x00007feeba9e19d6 in uuid_unparse_x (uu=<value optimized out>,
>> out=0x2350fc0 "081bbc7a-7551-44ac-85c7-aad5e2633db9",
>>     fmt=0x7feebaa08e00
>> "%08x-%04x-%04x-%02x%02x-%02x%02x%02x%02x%02x%02x") at
>> ../../contrib/uuid/unparse.c:55
>> #2  0x00007feeba9be837 in uuid_utoa (uuid=0x8 <Address 0x8 out of
>> bounds>) at common-utils.c:2138
>> #3  0x00007feeb06e8a58 in pl_inodelk_log_cleanup (this=0x230d910,
>> ctx=0x7fee700f0c60) at inodelk.c:396
>> #4  pl_inodelk_client_cleanup (this=0x230d910, ctx=0x7fee700f0c60) at
>> inodelk.c:428
>> #5  0x00007feeb06ddf3a in pl_client_disconnect_cbk (this=0x230d910,
>> client=<value optimized out>) at posix.c:2550
>> #6  0x00007feeba9fa2dd in gf_client_disconnect (client=0x27724a0) at
>> client_t.c:368
>> #7  0x00007feeab77ed48 in server_connection_cleanup (this=0x2316390,
>> client=0x27724a0, flags=<value optimized out>) at server-helpers.c:354
>> #8  0x00007feeab77ae2c in server_rpc_notify (rpc=<value optimized
>> out>, xl=0x2316390, event=<value optimized out>, data=0x2bf51c0) at
>> server.c:527
>> #9  0x00007feeba775155 in rpcsvc_handle_disconnect (svc=0x2325980,
>> trans=0x2bf51c0) at rpcsvc.c:720
>> #10 0x00007feeba776c30 in rpcsvc_notify (trans=0x2bf51c0,
>> mydata=<value optimized out>, event=<value optimized out>,
>> data=0x2bf51c0) at rpcsvc.c:758
>> #11 0x00007feeba778638 in rpc_transport_notify (this=<value optimized
>> out>, event=<value optimized out>, data=<value optimized out>) at
>> rpc-transport.c:512
>> #12 0x00007feeb115e971 in socket_event_poll_err (fd=<value optimized
>> out>, idx=<value optimized out>, data=0x2bf51c0, poll_in=<value
>> optimized out>, poll_out=0,
>>     poll_err=0) at socket.c:1071
>> #13 socket_event_handler (fd=<value optimized out>, idx=<value
>> optimized out>, data=0x2bf51c0, poll_in=<value optimized out>,
>> poll_out=0, poll_err=0) at socket.c:2240
>> #14 0x00007feeba9fc6a7 in event_dispatch_epoll_handler
>> (event_pool=0x22e2d00) at event-epoll.c:384
>> #15 event_dispatch_epoll (event_pool=0x22e2d00) at event-epoll.c:445
>> #16 0x0000000000407e93 in main (argc=19, argv=0x7fffea6c7f88) at
>> glusterfsd.c:2023
>> (gdb) f 4
>> #4  pl_inodelk_client_cleanup (this=0x230d910, ctx=0x7fee700f0c60) at
>> inodelk.c:428
>> 428                pl_inodelk_log_cleanup (l);
>> (gdb) p l->pl_inode->refkeeper
>> $1 = (inode_t *) 0x0
>> (gdb)
>>
>> pl_inode->refkeeper was found to be NULL even when there were some
>> blocked inodelks in a certain domain of the inode,
>> which when dereferenced by the epoll thread in the cleanup codepath
>> led to a crash.
>>
>> On inspecting the code (for want of a consistent reproducer), three
>> things were found:
>>
>> 1. The function where the crash happens (pl_inodelk_log_cleanup()),
>> makes an attempt to resolve the inode to path as can be seen below.
>> But the way inode_path() itself
>>     works is to first construct the path based on the given inode's
>> ancestry and place it in the buffer provided. And if all else fails,
>> the gfid of the inode is placed in a certain format ("<gfid:%s>").
>>     This eliminates the need for statements from line 4 through 7
>> below, thereby "preventing" dereferencing of pl_inode->refkeeper.
>>     Now, although this change prevents the crash altogether, it still
>> does not fix the race that led to pl_inode->refkeeper becoming NULL,
>> and comes at the cost of
>>     printing "(null)" in the log message on line 9 every time
>> pl_inode->refkeeper is found to be NULL, rendering the logged messages
>> somewhat useless.
>>
>> <code>
>>   0         pl_inode = lock->pl_inode;
>>   1
>>   2         inode_path (pl_inode->refkeeper, NULL, &path);
>>   3
>>   4         if (path)
>>   5                 file = path;
>>   6         else
>>   7                 file = uuid_utoa (pl_inode->refkeeper->gfid);
>> 8
>>   9         gf_log (THIS->name, GF_LOG_WARNING,
>>  10                 "releasing lock on %s held by "
>>  11                 "{client=%p, pid=%"PRId64" lk-owner=%s}",
>>  12                 file, lock->client, (uint64_t) lock->client_pid,
>>  13                 lkowner_utoa (&lock->owner));
>> <\code>
> I think this logging code is from the days when gfid handle concept was
> not there. So it wasn't returning <gfid:gfid-str> in cases the path is
> not present in the dentries. I believe the else block can be deleted
> safely now.
>
> Pranith
>>
>> 2. There is at least one codepath found that can lead to this crash:
>>     Imagine an inode on which an inodelk operation is attempted by a
>> client and is successfully granted too.
>>    Now, between the time the lock was granted and
>> pl_update_refkeeper() was called by this thread, the client could send
>> a DISCONNECT event,
>>    causing cleanup codepath to be executed, where the epoll thread
>> crashes on dereferencing pl_inode->refkeeper which is STILL NULL at
>> this point.
>>
>>    Besides, there are still places in locks xlator where the refkeeper
>> is NOT updated whenever the lists are modified - for instance in the
>> cleanup codepath from a DISCONNECT.
>>
>> 3. Also, pl_update_refkeeper() seems to be not taking into account
>> blocked locks on the inode in the __pl_inode_is_empty() check, when it
>> should, as there could still be cases
>>     where the granted list could be empty but not the blocked list at
>> the time of udpating the refkeeper, in which case pl_inode must still
>> take ref on the inode.
> Avati,
>       There is one more issue, where pl_inode_forget is cleaning up
> blocked/granted locks which is not handling the new 'client_list'. But
> if we fix inode ref/unref like Kritika proposed, pl_forget should never
> come when there is a granted/blocked lock. So may be we should delete
> that as well. Please feel free to comment.
>

Actually any kind of cleaning up in pl_inode_forget() is a gimmick. 
refkeeper guarantees that the inode is not forgotten when a lock is 
held. I don't think Kritika's proposed changes are necessary for it to 
be true. The NULL pointer encountered in the bug is because of the abuse 
of refkeeper interface to get hold of the GFID. Even with things left 
as-is, we should never have any left over locks or blocked locks in 
pl_forget().

Thanks