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

[Krutika Dhananjay]

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> 

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. 

Proposed solution to 2/3: 

1. Do away with refkeeper in pl_inode_t altogether. 
2. Let every lock object (inodelk/entryllk/posix_lock) have an inode_t * member to act as a placeholder for the associated inode object on which it is locking. 
3. Let each lock object hold a ref on the inode at the time of its creation and unref the inode during its destruction. 

Please let me know what you think of the above. 

-Krutika 


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