[PATCH] block: add single-check guard in throttle_group_restart_queue to address race with schedule_next_request

[luzhipeng <luzhipeng@cestc.cn>]

A race condition exists between throttle_group_restart_queue() and
schedule_next_request(): when multiple ThrottleGroupMembers in the same
throttle group are assigned to different IOThreads, concurrent execution
can cause schedule_next_request() to re-arm a throttle timer while
throttle_group_restart_queue() is being called (e.g., from a timer
callback or external restart). This violates the assumption that no
timer is pending upon entry to throttle_group_restart_queue(), triggering
an assertion failure and causing QEMU to abort.

This patch replaces the assert with a single early-return check:
if the timer for the given direction is already pending, the function
returns immediately. This prevents duplicate coroutine scheduling and
avoids crashes under race conditions, without altering the core
(non-thread-safe) throttle group logic.

For details, see: https://gitlab.com/qemu-project/qemu/-/issues/3194

Signed-off-by: luzhipeng <luzhipeng@cestc.cn>
---
 block/throttle-groups.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/block/throttle-groups.c b/block/throttle-groups.c
index 66fdce9a90..9dcc6b4923 100644
--- a/block/throttle-groups.c
+++ b/block/throttle-groups.c
@@ -430,15 +430,14 @@ static void throttle_group_restart_queue(ThrottleGroupMember *tgm,
                                         ThrottleDirection direction)
 {
     Coroutine *co;
+    if (timer_pending(tgm->throttle_timers.timers[direction])) {
+        return;
+    }
     RestartData *rd = g_new0(RestartData, 1);
 
     rd->tgm = tgm;
     rd->direction = direction;
 
-    /* This function is called when a timer is fired or when
-     * throttle_group_restart_tgm() is called. Either way, there can
-     * be no timer pending on this tgm at this point */
-    assert(!timer_pending(tgm->throttle_timers.timers[direction]));
 
     qatomic_inc(&tgm->restart_pending);
 
-- 
2.31.1