[Gluster-devel] libgfapi changes to add lk_owner and lease ID

[Ira Cooper]

Niels de Vos <ndevos at redhat.com> writes:

> On Thu, Dec 03, 2015 at 06:34:56PM -0500, Ira Cooper wrote:
>> Kaleb KEITHLEY <kkeithle at redhat.com> writes:
>> 
>> > On 12/03/2015 03:10 PM, Ira Cooper wrote:
>> >> Poornima Gurusiddaiah <pgurusid at redhat.com> writes:
>> >> 
>> >>> Hi, 
>> >>>
>> >>> Brief Background: 
>> >>> ============ 
>> >>> For the below two features, we need ligfapi to take 2 other parameters from the applications for most number of fops. 
>> >>> http://review.gluster.org/#/c/12014/ 
>> >>> http://review.gluster.org/#/c/11980/ 
>> >>>
>> >>> For leases to work as explained in the above doc, every file data read/write fop needs to be associated with a lease ID. This is specially required for Samba and NFS-Ganesha as they inturn serve other clients who request for leases. For Gluster to identify the end client (which is not Samba/NFS Ganesha) we need lease ID to be filled by Samba/NFS Ganesha. 
>> >>>
>> >>> For mandatory locks feature to work as explained, every file data read/write fop needs to be associated with a lk_owner. In linux Kernel VFS takes care of filling the lk_ownere for the file system. In libgfapi case, the applications calling into libgfapi should be providing lk_owner with every fop. This is again required mainly for Samba and NFS Ganesha, as they serve multiple clients. 
>> >>>
>> >>> Possible solutions: 
>> >>> ============= 
>> >>> 1. Modify all the required APIs to take 2 other parameter, lease ID and lk_owner. But that would mean backward compatibility issues and is a programming overhead for applications not interested in Leases and mandatory lock feature. 
>> >>> 2. Add an API called glfs_set_fop_attrs (lease ID, lk_owner) which works similar to glfs_set_uid(uid). The API sets a thread local storage (pthread_key) with the values provided, the further fops on that thread will pick the lease ID and lk_owner from the thread local storage (pthread_key). There are few minor details that needs to be worked out: 
>> >>> - In case of async API will end up using lease ID and lk_owner from wrong thread. 
>> >>> - unset lease ID and lk_owner after every fop to ensure there is no stale lease ID or lk_owner set? 
>> >>> - For fd based fops we can store the lease ID and lk_owner in glfd, so that the application writed need not set it for every fop. But for handle based fops lease ID and lk_owner needs to be set explicitly every-time. 
>> >>>
>> >>> Solution 2 is more preferable except for that it adds overhead of calling another API, for the libgfapi users who intends to use these features. 
>> >>> A prototype of solution 2 can be found at http://review.gluster.org/#/c/12876/ 
>> >>>
>> >
>> > why not use storage in the client_t instead of thread local?
>> 
>> It comes down to the use case.  For Samba, the right spot is almost
>> certainly the fd, because lease keys are a per-handle (which we map to
>> fd) property.
>> 
>> client_t is a horror show, due to race conditions between threads, IMHO.
>
> If there are known races, should we not address that? Got a bug that
> explains it in more detail?

Niels,

For samba, if we do multi-threaded open, Kaleb's proposal is a
race-condition.  I haven't gone through every use of client_t and seen
if it is racy.

The race here is pretty simple:

Thread 1: Sets lease_id
Thread 2: Sets lease_id
Thread 1: Opens file. (wrong lease_id)

If these two threads represent requests from different clients, client_t
won't work, unless there's a client_t per-thread.

For global things on the connection, client_t is fine, and appropriate.
For this?  No.

This is a property per-open, and belongs in the glfs_fd and glfs_object,
IMHO.

Thanks,

-Ira