staging: lustre: obdclass: fix race during key quiescency

Upon umount, presumably of last device using same OSD back-end,
to prepare for module unload, lu_context_key_quiesce() is run to
remove all module's key reference in any context linked on
lu_context_remembered list.
Threads must protect against such transversal processing when
exiting from its context.
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5264
Reviewed-on: http://review.whamcloud.com/13103Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/obdclass/lu_object.c

@@ -1663,6 +1663,9 @@ void lu_context_exit(struct lu_context *ctx)
 	ctx->lc_state = LCS_LEFT;
 	if (ctx->lc_tags & LCT_HAS_EXIT && ctx->lc_value) {
 		for (i = 0; i < ARRAY_SIZE(lu_keys); ++i) {
+			/* could race with key quiescency */
+			if (ctx->lc_tags & LCT_REMEMBER)
+				spin_lock(&lu_keys_guard);
 			if (ctx->lc_value[i]) {
 				struct lu_context_key *key;
 
@@ -1671,6 +1674,8 @@ void lu_context_exit(struct lu_context *ctx)
 					key->lct_exit(ctx,
 						      key, ctx->lc_value[i]);
 			}
+			if (ctx->lc_tags & LCT_REMEMBER)
+				spin_unlock(&lu_keys_guard);
 		}
 	}
 }