9pfs: fix FD leak and reduce latency of v9fs_reclaim_fd()

This patch fixes two different bugs in v9fs_reclaim_fd():

1. Reduce latency:

This function calls v9fs_co_close() and v9fs_co_closedir() in a loop. Each
one of the calls adds two thread hops (between main thread and a fs driver
background thread). Each thread hop adds latency, which sums up in
function's loop to a significant duration.

Reduce overall latency by open coding what v9fs_co_close() and
v9fs_co_closedir() do, executing those and the loop itself altogether in
only one background thread block, hence reducing the total amount of
thread hops to only two.

2. Fix file descriptor leak:

The existing code called v9fs_co_close() and v9fs_co_closedir() to close
file descriptors. Both functions check right at the beginning if the 9p
request was cancelled:

    if (v9fs_request_cancelled(pdu)) {
        return -EINTR;
    }

So if client sent a 'Tflush' message, v9fs_co_close() / v9fs_co_closedir()
returned without having closed the file descriptor and v9fs_reclaim_fd()
subsequently freed the FID without its file descriptor being closed, hence
leaking those file descriptors.

This 2nd bug is fixed by this patch as well by open coding v9fs_co_close()
and v9fs_co_closedir() inside of v9fs_reclaim_fd() and not performing the
v9fs_request_cancelled(pdu) check there.

Fixes: 7a46274529 ('hw/9pfs: Add file descriptor reclaim support')
Fixes: bccacf6c79 ('hw/9pfs: Implement TFLUSH operation')
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <5747469d3f039c53147e850b456943a1d4b5485c.1741339452.git.qemu_oss@crudebyte.com>

hw/9pfs/9p.c

@@ -434,6 +434,8 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
     V9fsFidState *f;
     GHashTableIter iter;
     gpointer fid;
+    int err;
+    int nclosed = 0;
 
     /* prevent multiple coroutines running this function simultaniously */
     if (s->reclaiming) {
@@ -446,10 +448,10 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
     QSLIST_HEAD(, V9fsFidState) reclaim_list =
         QSLIST_HEAD_INITIALIZER(reclaim_list);
 
+    /* Pick FIDs to be closed, collect them on reclaim_list. */
     while (g_hash_table_iter_next(&iter, &fid, (gpointer *) &f)) {
         /*
-         * Unlink fids cannot be reclaimed. Check
+         * Unlinked fids cannot be reclaimed, skip those, and also skip fids
-         * for them and skip them. Also skip fids
          * currently being operated on.
          */
         if (f->ref || f->flags & FID_NON_RECLAIMABLE) {
@@ -499,17 +501,26 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
         }
     }
     /*
-     * Now close the fid in reclaim list. Free them if they
+     * Close the picked FIDs altogether on a background I/O driver thread. Do
-     * are already clunked.
+     * this all at once to keep latency (i.e. amount of thread hops between main
+     * thread <-> fs driver background thread) as low as possible.
      */
+    v9fs_co_run_in_worker({
+        QSLIST_FOREACH(f, &reclaim_list, reclaim_next) {
+            err = (f->fid_type == P9_FID_DIR) ?
+                s->ops->closedir(&s->ctx, &f->fs_reclaim) :
+                s->ops->close(&s->ctx, &f->fs_reclaim);
+            if (!err) {
+                /* total_open_fd must only be mutated on main thread */
+                nclosed++;
+            }
+        }
+    });
+    total_open_fd -= nclosed;
+    /* Free the closed FIDs. */
     while (!QSLIST_EMPTY(&reclaim_list)) {
         f = QSLIST_FIRST(&reclaim_list);
         QSLIST_REMOVE(&reclaim_list, f, V9fsFidState, reclaim_next);
-        if (f->fid_type == P9_FID_FILE) {
-            v9fs_co_close(pdu, &f->fs_reclaim);
-        } else if (f->fid_type == P9_FID_DIR) {
-            v9fs_co_closedir(pdu, &f->fs_reclaim);
-        }
         /*
          * Now drop the fid reference, free it
          * if clunked.