From: Chris Caputo <[EMAIL PROTECTED]>
[PATCH 2.6.19-rc6] sunrpc: fix race condition
Patch linux-2.6.10-01-rpc_workqueue.dif introduced a race condition into
net/sunrpc/sched.c in kernels 2.6.11-rc1 through 2.6.19-rc6. The race
scenario is as follows...
Given: RPC_TASK_QUEUED, RPC_TASK_RUNNING and RPC_TASK_ASYNC are set.
__rpc_execute() (no spinlock) rpc_make_runnable() (queue spinlock held)
----------------------------- -----------------------------------------
do_ret = rpc_test_and_set_running(task);
rpc_clear_running(task);
if (RPC_IS_ASYNC(task)) {
if (RPC_IS_QUEUED(task))
return 0;
rpc_clear_queued(task);
if (do_ret)
return;
Thus both threads return and the task is abandoned forever.
In my test NFS client usage (~200 Mb/s at ~3,000 RPC calls/s) this race
condition has resulted in processes getting permanently stuck in 'D' state
often in less than 15 minutes of uptime.
The following patch fixes the problem by returning to use of a spinlock in
__rpc_execute().
Signed-off-by: Chris Caputo <[EMAIL PROTECTED]>
---
diff -up a/net/sunrpc/sched.c b/net/sunrpc/sched.c
--- a/net/sunrpc/sched.c 2006-11-27 08:41:07.000000000 +0000
+++ b/net/sunrpc/sched.c 2006-11-27 11:14:21.000000000 +0000
@@ -587,6 +587,7 @@ EXPORT_SYMBOL(rpc_exit_task);
static int __rpc_execute(struct rpc_task *task)
{
int status = 0;
+ struct rpc_wait_queue *queue;
dprintk("RPC: %4d rpc_execute flgs %x\n",
task->tk_pid, task->tk_flags);
@@ -631,22 +632,27 @@ static int __rpc_execute(struct rpc_task
lock_kernel();
task->tk_action(task);
unlock_kernel();
+ /* micro-optimization to avoid spinlock */
+ if (!RPC_IS_QUEUED(task))
+ continue;
}
/*
- * Lockless check for whether task is sleeping or not.
+ * Check whether task is sleeping.
*/
- if (!RPC_IS_QUEUED(task))
- continue;
- rpc_clear_running(task);
+ queue = task->u.tk_wait.rpc_waitq;
+ spin_lock_bh(&queue->lock);
if (RPC_IS_ASYNC(task)) {
- /* Careful! we may have raced... */
- if (RPC_IS_QUEUED(task))
- return 0;
- if (rpc_test_and_set_running(task))
+ if (RPC_IS_QUEUED(task)) {
+ rpc_clear_running(task);
+ spin_unlock_bh(&queue->lock);
return 0;
+ }
+ spin_unlock_bh(&queue->lock);
continue;
}
+ rpc_clear_running(task);
+ spin_unlock_bh(&queue->lock);
/* sync task: sleep here */
dprintk("RPC: %4d sync task going to sleep\n", task->tk_pid);
-
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