linux-kernel - [ 26/49] SUNRPC: Ensure that we free the rpc

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Message-Id: <20130113174301.254522617@decadent.org.uk>
Date:	Sun, 13 Jan 2013 17:43:21 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Weston Andros Adamson <dros@...app.com>,
	Tejun Heo <tj@...nel.org>, Bruce Fields <bfields@...ldses.org>
Subject: [ 26/49] SUNRPC: Ensure that we free the rpc_task after cleanups are done

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Trond Myklebust <Trond.Myklebust@...app.com>

commit c6567ed1402c55e19b012e66a8398baec2a726f3 upstream.

This patch ensures that we free the rpc_task after the cleanup callbacks
are done in order to avoid a deadlock problem that can be triggered if
the callback needs to wait for another workqueue item to complete.

Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
Cc: Weston Andros Adamson <dros@...app.com>
Cc: Tejun Heo <tj@...nel.org>
Cc: Bruce Fields <bfields@...ldses.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/sunrpc/sched.c |   27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index d17a704..b4133bd 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -934,16 +934,35 @@ struct rpc_task *rpc_new_task(const struct rpc_task_setup *setup_data)
 	return task;
 }
 
+/*
+ * rpc_free_task - release rpc task and perform cleanups
+ *
+ * Note that we free up the rpc_task _after_ rpc_release_calldata()
+ * in order to work around a workqueue dependency issue.
+ *
+ * Tejun Heo states:
+ * "Workqueue currently considers two work items to be the same if they're
+ * on the same address and won't execute them concurrently - ie. it
+ * makes a work item which is queued again while being executed wait
+ * for the previous execution to complete.
+ *
+ * If a work function frees the work item, and then waits for an event
+ * which should be performed by another work item and *that* work item
+ * recycles the freed work item, it can create a false dependency loop.
+ * There really is no reliable way to detect this short of verifying
+ * every memory free."
+ *
+ */
 static void rpc_free_task(struct rpc_task *task)
 {
-	const struct rpc_call_ops *tk_ops = task->tk_ops;
-	void *calldata = task->tk_calldata;
+	unsigned short tk_flags = task->tk_flags;
+
+	rpc_release_calldata(task->tk_ops, task->tk_calldata);
 
-	if (task->tk_flags & RPC_TASK_DYNAMIC) {
+	if (tk_flags & RPC_TASK_DYNAMIC) {
 		dprintk("RPC: %5u freeing task\n", task->tk_pid);
 		mempool_free(task, rpc_task_mempool);
 	}
-	rpc_release_calldata(tk_ops, calldata);
 }
 
 static void rpc_async_release(struct work_struct *work)


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