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SubjectRe: [PATCH] SUNRPC: Allow one callback request to be received from two sk_buff
From
2014/1/21 Trond Myklebust <trond.myklebust@primarydata.com>:
> On Mon, 2014-01-20 at 14:59 +0800, shaobingqing wrote:
>> In current code, there only one struct rpc_rqst is prealloced. If one
>> callback request is received from two sk_buff, the xprt_alloc_bc_request
>> would be execute two times with the same transport->xid. The first time
>> xprt_alloc_bc_request will alloc one struct rpc_rqst and the TCP_RCV_COPY_DATA
>> bit of transport->tcp_flags will not be cleared. The second time
>> xprt_alloc_bc_request could not alloc struct rpc_rqst any more and NULL
>> pointer will be returned, then xprt_force_disconnect occur. I think one
>> callback request can be allowed to be received from two sk_buff.
>>
>> Signed-off-by: shaobingqing <shaobingqing@bwstor.com.cn>
>> ---
>> net/sunrpc/xprtsock.c | 11 +++++++++--
>> 1 files changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
>> index ee03d35..606950d 100644
>> --- a/net/sunrpc/xprtsock.c
>> +++ b/net/sunrpc/xprtsock.c
>> @@ -1271,8 +1271,13 @@ static inline int xs_tcp_read_callback(struct rpc_xprt *xprt,
>> struct sock_xprt *transport =
>> container_of(xprt, struct sock_xprt, xprt);
>> struct rpc_rqst *req;
>> + static struct rpc_rqst *req_partial;
>> +
>> + if (req_partial == NULL)
>> + req = xprt_alloc_bc_request(xprt);
>> + else if (req_partial->rq_xid == transport->tcp_xid)
>> + req = req_partial;
>
> What happens here if req_partial->rq_xid != transport->tcp_xid? AFAICS,
> req will be undefined. Either way, you cannot use a static variable for
> storage here: that isn't re-entrant.

Because metadata sever only have one slot for backchannel request,
req_partial->rq_xid == transport->tcp_xid always happens, if the callback
request just being splited in two sk_buffs. But req_partial->rq_xid !=
transport->tcp_xid may also happens in some special cases, such as
retransmission occurs?
If one callback request is splited in two sk_buffs, xs_tcp_read_callback
will be execute two times. The req_partial should be a static variable,
because the second execution of xs_tcp_read_callback should use
the rpc_rqst allocated for the first execution, which saves information
copies from the first sk_buff.
I think perhaps the code should be modified like bellows:

diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index 606950d..02dbb82 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c
@@ -1273,10 +1273,14 @@ static inline int xs_tcp_read_callback(struct
rpc_xprt *xprt,
struct rpc_rqst *req;
static struct rpc_rqst *req_partial;

- if (req_partial == NULL)
+ if (req_partial == NULL) {
req = xprt_alloc_bc_request(xprt);
- else if (req_partial->rq_xid == transport->tcp_xid)
+ } else if (req_partial->rq_xid == transport->tcp_xid) {
req = req_partial;
+ } else {
+ xprt_free_bc_request(req_partial);
+ req = xprt_alloc_bc_request(xprt);
+ }

if (req == NULL) {
printk(KERN_WARNING "Callback slot table overflowed\n");
@@ -1303,8 +1307,9 @@ static inline int xs_tcp_read_callback(struct
rpc_xprt *xprt,
list_add(&req->rq_bc_list, &bc_serv->sv_cb_list);
spin_unlock(&bc_serv->sv_cb_lock);
wake_up(&bc_serv->sv_cb_waitq);
- } else
+ } else {
req_partial = req;
+ }

req->rq_private_buf.len = transport->tcp_copied;

>
>> - req = xprt_alloc_bc_request(xprt);
>> if (req == NULL) {
>> printk(KERN_WARNING "Callback slot table overflowed\n");
>> xprt_force_disconnect(xprt);
>> @@ -1285,6 +1290,7 @@ static inline int xs_tcp_read_callback(struct rpc_xprt *xprt,
>>
>> if (!(transport->tcp_flags & TCP_RCV_COPY_DATA)) {
>> struct svc_serv *bc_serv = xprt->bc_serv;
>> + req_partial = NULL;
>>
>> /*
>> * Add callback request to callback list. The callback
>> @@ -1297,7 +1303,8 @@ static inline int xs_tcp_read_callback(struct rpc_xprt *xprt,
>> list_add(&req->rq_bc_list, &bc_serv->sv_cb_list);
>> spin_unlock(&bc_serv->sv_cb_lock);
>> wake_up(&bc_serv->sv_cb_waitq);
>> - }
>> + } else
>> + req_partial = req;
>>
>> req->rq_private_buf.len = transport->tcp_copied;
>>
>
>
> --
> Trond Myklebust
> Linux NFS client maintainer
>
> --
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