On Thu, Jul 02, 2015 at 05:43:32PM +0100, Al Viro wrote:
>       req->tc->tag = tag-1;
> +     if (req->status != REQ_STATUS_IDLE)
> +             pr_err("using tag %d with odd status (%d)", tag, req->status);

Should be tag - 1 here, actually.

diff --git a/net/9p/client.c b/net/9p/client.c
index 6f4c4c8..9719886 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -248,6 +248,9 @@ p9_tag_alloc(struct p9_client *c, u16 tag, unsigned int 
max_size)
                /* check again since original check was outside of lock */
                while (tag >= c->max_tag) {
                        row = (tag / P9_ROW_MAXTAG);
+
+                       WARN_ON_ONCE(c->reqs[row]);     // are we about to leak?
+
                        c->reqs[row] = kcalloc(P9_ROW_MAXTAG,
                                        sizeof(struct p9_req_t), GFP_ATOMIC);
 
@@ -286,6 +289,8 @@ p9_tag_alloc(struct p9_client *c, u16 tag, unsigned int 
max_size)
        p9pdu_reset(req->rc);
 
        req->tc->tag = tag-1;
+       if (req->status != REQ_STATUS_IDLE)
+               pr_err("using tag %d with odd status (%d)", tag - 1, 
req->status);
        req->status = REQ_STATUS_ALLOC;
 
        return req;
@@ -425,6 +430,8 @@ void p9_client_cb(struct p9_client *c, struct p9_req_t 
*req, int status)
         * the other thread wakes up will indeed be seen by the waiting side.
         */
        smp_wmb();
+       if (req->status == REQ_STATUS_IDLE)
+               pr_err("late delivery, tag %d already freed", req->tc->tag);
        req->status = status;
 
        wake_up(req->wq);
@@ -693,6 +700,10 @@ static struct p9_req_t *p9_client_prepare_req(struct 
p9_client *c,
                tag = p9_idpool_get(c->tagpool);
                if (tag < 0)
                        return ERR_PTR(-ENOMEM);
+               if (WARN_ON_ONCE(tag != (u16)tag)) {    // wrapped around?
+                       p9_idpool_put(tag, c->tagpool);
+                       return ERR_PTR(-ENOMEM);
+               }
        }
 
        req = p9_tag_alloc(c, tag, req_size);
@@ -1647,7 +1658,10 @@ p9_client_write(struct p9_fid *fid, u64 offset, struct 
iov_iter *from, int *err)
                if (*err) {
                        trace_9p_protocol_dump(clnt, req->rc);
                        p9_free_req(clnt, req);
+                       break;
                }
+               if (rsize < count)
+                       pr_err("mismatched reply [tag = %d]\n", req->tc->tag);
 
                p9_debug(P9_DEBUG_9P, "<<< RWRITE count %d\n", count);
 
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