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staging: lustre: mdc: document mdc_rpc_lock

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As this lock can be a bottleneck, clarifying why it is needed may be
helpful to those working on client performance.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3443
Reviewed-on: http://review.whamcloud.com/6593
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ned Bass 2016-04-04 21:36:57 -04:00 committed by Greg Kroah-Hartman
parent 46dfb5aa98
commit 6a1938d958
1 changed files with 18 additions and 0 deletions
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18
drivers/staging/lustre/lustre/include/lustre_mdc.h
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@ -64,9 +64,27 @@ struct obd_export;
struct ptlrpc_request;
struct obd_device;
/**
* Serializes in-flight MDT-modifying RPC requests to preserve idempotency.
*
* This mutex is used to implement execute-once semantics on the MDT.
* The MDT stores the last transaction ID and result for every client in
* its last_rcvd file. If the client doesn't get a reply, it can safely
* resend the request and the MDT will reconstruct the reply being aware
* that the request has already been executed. Without this lock,
* execution status of concurrent in-flight requests would be
* overwritten.
*
* This design limits the extent to which we can keep a full pipeline of
* in-flight requests from a single client. This limitation could be
* overcome by allowing multiple slots per client in the last_rcvd file.
*/
struct mdc_rpc_lock {
/** Lock protecting in-flight RPC concurrency. */
struct mutex rpcl_mutex;
/** Intent associated with currently executing request. */
struct lookup_intent *rpcl_it;
/** Used for MDS/RPC load testing purposes. */
int rpcl_fakes;
};