nfsd: prevent callback tasks running concurrently

The nfsd4_callback workqueue jobs exist to queue backchannel RPCs to
rpciod. Because they run in different workqueue contexts, the rpc_task
can run concurrently with the workqueue job itself, should it become
requeued. This is problematic as there is no locking when accessing the
fields in the nfsd4_callback.

Add a new unsigned long to nfsd4_callback and declare a new
NFSD4_CALLBACK_RUNNING flag to be set in it. When attempting to run a
workqueue job, do a test_and_set_bit() on that flag first, and don't
queue the workqueue job if it returns true. Clear NFSD4_CALLBACK_RUNNING
in nfsd41_destroy_cb().

This also gives us a more reliable mechanism for handling queueing
failures in codepaths where we have to take references under spinlocks.
We can now do the test_and_set_bit on NFSD4_CALLBACK_RUNNING first, and
only take references to the objects if that returns false.

Most of the nfsd4_run_cb() callers are converted to use this new flag or
the nfsd4_try_run_cb() wrapper. The main exception is the callback
channel probe, which has its own synchronization.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4callback.c

@@ -1312,6 +1312,7 @@ static void nfsd41_destroy_cb(struct nfsd4_callback *cb)
 
 	trace_nfsd_cb_destroy(clp, cb);
 	nfsd41_cb_release_slot(cb);
+	clear_bit(NFSD4_CALLBACK_RUNNING, &cb->cb_flags);
 	if (cb->cb_ops && cb->cb_ops->release)
 		cb->cb_ops->release(cb);
 	nfsd41_cb_inflight_end(clp);
@@ -1632,6 +1633,7 @@ void nfsd4_init_cb(struct nfsd4_callback *cb, struct nfs4_client *clp,
 	cb->cb_msg.rpc_proc = &nfs4_cb_procedures[op];
 	cb->cb_msg.rpc_argp = cb;
 	cb->cb_msg.rpc_resp = cb;
+	cb->cb_flags = 0;
 	cb->cb_ops = ops;
 	INIT_WORK(&cb->cb_work, nfsd4_run_cb_work);
 	cb->cb_status = 0;

fs/nfsd/nfs4layouts.c

@@ -344,9 +344,10 @@ nfsd4_recall_file_layout(struct nfs4_layout_stateid *ls)
 	atomic_inc(&ls->ls_stid.sc_file->fi_lo_recalls);
 	trace_nfsd_layout_recall(&ls->ls_stid.sc_stateid);
 
-	refcount_inc(&ls->ls_stid.sc_count);
+	if (!test_and_set_bit(NFSD4_CALLBACK_RUNNING, &ls->ls_recall.cb_flags)) {
-	nfsd4_run_cb(&ls->ls_recall);
+		refcount_inc(&ls->ls_stid.sc_count);
-
+		nfsd4_run_cb(&ls->ls_recall);
+	}
 out_unlock:
 	spin_unlock(&ls->ls_lock);
 }

fs/nfsd/nfs4proc.c

@@ -1847,7 +1847,7 @@ static void nfsd4_send_cb_offload(struct nfsd4_copy *copy)
 		      NFSPROC4_CLNT_CB_OFFLOAD);
 	trace_nfsd_cb_offload(copy->cp_clp, &cbo->co_res.cb_stateid,
 			      &cbo->co_fh, copy->cp_count, copy->nfserr);
-	nfsd4_run_cb(&cbo->co_cb);
+	nfsd4_try_run_cb(&cbo->co_cb);
 }
 
 /**

fs/nfsd/nfs4state.c

@@ -3232,8 +3232,10 @@ static void nfs4_cb_getattr(struct nfs4_cb_fattr *ncf)
 	/* set to proper status when nfsd4_cb_getattr_done runs */
 	ncf->ncf_cb_status = NFS4ERR_IO;
 
-	refcount_inc(&dp->dl_stid.sc_count);
+	if (!test_and_set_bit(NFSD4_CALLBACK_RUNNING, &ncf->ncf_getattr.cb_flags)) {
-	nfsd4_run_cb(&ncf->ncf_getattr);
+		refcount_inc(&dp->dl_stid.sc_count);
+		nfsd4_run_cb(&ncf->ncf_getattr);
+	}
 }
 
 static struct nfs4_client *create_client(struct xdr_netobj name,
@@ -5422,6 +5424,10 @@ static const struct nfsd4_callback_ops nfsd4_cb_recall_ops = {
 static void nfsd_break_one_deleg(struct nfs4_delegation *dp)
 {
 	bool queued;
+
+	if (test_and_set_bit(NFSD4_CALLBACK_RUNNING, &dp->dl_recall.cb_flags))
+		return;
+
 	/*
 	 * We're assuming the state code never drops its reference
 	 * without first removing the lease.  Since we're in this lease
@@ -6910,7 +6916,7 @@ deleg_reaper(struct nfsd_net *nn)
 		clp->cl_ra->ra_bmval[0] = BIT(RCA4_TYPE_MASK_RDATA_DLG) |
 						BIT(RCA4_TYPE_MASK_WDATA_DLG);
 		trace_nfsd_cb_recall_any(clp->cl_ra);
-		nfsd4_run_cb(&clp->cl_ra->ra_cb);
+		nfsd4_try_run_cb(&clp->cl_ra->ra_cb);
 	}
 }
 
@@ -7839,7 +7845,7 @@ nfsd4_lm_notify(struct file_lock *fl)
 
 	if (queue) {
 		trace_nfsd_cb_notify_lock(lo, nbl);
-		nfsd4_run_cb(&nbl->nbl_cb);
+		nfsd4_try_run_cb(&nbl->nbl_cb);
 	}
 }
 

fs/nfsd/state.h

@@ -67,6 +67,8 @@ typedef struct {
 struct nfsd4_callback {
 	struct nfs4_client *cb_clp;
 	struct rpc_message cb_msg;
+#define NFSD4_CALLBACK_RUNNING		(0)
+	unsigned long cb_flags;
 	const struct nfsd4_callback_ops *cb_ops;
 	struct work_struct cb_work;
 	int cb_seq_status;
@@ -780,6 +782,13 @@ extern void nfsd4_change_callback(struct nfs4_client *clp, struct nfs4_cb_conn *
 extern void nfsd4_init_cb(struct nfsd4_callback *cb, struct nfs4_client *clp,
 		const struct nfsd4_callback_ops *ops, enum nfsd4_cb_op op);
 extern bool nfsd4_run_cb(struct nfsd4_callback *cb);
+
+static inline void nfsd4_try_run_cb(struct nfsd4_callback *cb)
+{
+	if (!test_and_set_bit(NFSD4_CALLBACK_RUNNING, &cb->cb_flags))
+		WARN_ON_ONCE(!nfsd4_run_cb(cb));
+}
+
 extern void nfsd4_shutdown_callback(struct nfs4_client *);
 extern void nfsd4_shutdown_copy(struct nfs4_client *clp);
 void nfsd4_async_copy_reaper(struct nfsd_net *nn);