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

Pranith Kumar Karampuri pkarampu at redhat.com
Wed Jun 4 06:32:10 UTC 2014


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.

Pranith
>
> 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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