[Gluster-devel] Regarding doing away with refkeeper in locks xlator
Pranith Kumar Karampuri
pkarampu at redhat.com
Wed Jun 4 06:53:59 UTC 2014
On 06/04/2014 12:02 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.
It is pl_forget not pl_inode_forget. typo :-(.
Pranith
>
> 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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>
>
>
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