9pfs: fix concurrent v9fs_reclaim_fd() calls

Even though this function is serialized to be always called from main
thread, v9fs_reclaim_fd() is dispatching the coroutine to a worker thread
in between via its v9fs_co_*() calls, hence leading to the situation where
v9fs_reclaim_fd() is effectively executed multiple times simultaniously,
which renders its LRU algorithm useless and causes high latency.

Fix this by adding a simple boolean variable to ensure this function is
only called once at a time. No synchronization needed for this boolean
variable as this function is only entered and returned on main thread.

Fixes: 7a46274529c ('hw/9pfs: Add file descriptor reclaim support')
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <5c622067efd66dd4ee5eca740dcf263f41db20b2.1741339452.git.qemu_oss@crudebyte.com>
(cherry picked from commit 61da38db70affd925226ce1e8a61d761c20d045b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/9pfs/9p.c

@@ -435,6 +435,12 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
     GHashTableIter iter;
     gpointer fid;
 
+    /* prevent multiple coroutines running this function simultaniously */
+    if (s->reclaiming) {
+        return;
+    }
+    s->reclaiming = true;
+
     g_hash_table_iter_init(&iter, s->fids);
 
     QSLIST_HEAD(, V9fsFidState) reclaim_list =
@@ -510,6 +516,8 @@ void coroutine_fn v9fs_reclaim_fd(V9fsPDU *pdu)
          */
         put_fid(pdu, f);
     }
+
+    s->reclaiming = false;
 }
 
 /*
@@ -4324,6 +4332,8 @@ int v9fs_device_realize_common(V9fsState *s, const V9fsTransport *t,
     s->ctx.fst = &fse->fst;
     fsdev_throttle_init(s->ctx.fst);
 
+    s->reclaiming = false;
+
     rc = 0;
 out:
     if (rc) {

hw/9pfs/9p.h

@@ -362,6 +362,7 @@ struct V9fsState {
     uint64_t qp_ndevices; /* Amount of entries in qpd_table. */
     uint16_t qp_affix_next;
     uint64_t qp_fullpath_next;
+    bool reclaiming;
 };
 
 /* 9p2000.L open flags */