* [PATCH v3 1/1] throttle-group: Fix race condition in throttle_group_restart_queue()
2026-03-12 12:11 [PATCH v3 0/1] Fix race condition in throttle_group_restart_queue() Alberto Garcia
@ 2026-03-12 12:12 ` Alberto Garcia
2026-03-12 19:44 ` Jorge Merlino
2026-03-16 16:36 ` [PATCH v3 0/1] " Hanna Czenczek
1 sibling, 1 reply; 4+ messages in thread
From: Alberto Garcia @ 2026-03-12 12:12 UTC (permalink / raw)
To: qemu-devel
Cc: Alberto Garcia, Jorge Merlino, Kevin Wolf, qemu-block,
Hanna Czenczek
When a timer is fired a pending I/O request is restarted and
tg->any_timer_armed is reset so other requests can be scheduled.
However we're resetting any_timer_armed first in timer_cb() before
the request is actually restarted, and there's a window between both
moments in which another thread can arm the same timer, hitting an
assertion in throttle_group_restart_queue().
This can be solved by deferring the reset of tg->any_timer_armed to
the moment when the queue is actually restarted, which is protected by
tg->lock, preventing other threads from arming the timer before that.
In addition to that, throttle_group_restart_tgm() is also updated to
hold tg->lock while the timer is being inspected. Here we consider
three different scenarios:
- If the tgm has a timer set, fire it immediately
- If another tgm has a timer set, restart the queue anyway
- If there is no timer set in this group then simulate a timer that
fires immediately, by setting tg->any_timer_armed in order to
prevent other threads from arming a timer in the meantime.
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3194
Signed-off-by: Alberto Garcia <berto@igalia.com>
---
block/throttle-groups.c | 79 +++++++++++++++++++++++++++++++----------
1 file changed, 60 insertions(+), 19 deletions(-)
diff --git a/block/throttle-groups.c b/block/throttle-groups.c
index 5329ff1fdb..4b1b1944c2 100644
--- a/block/throttle-groups.c
+++ b/block/throttle-groups.c
@@ -391,6 +391,7 @@ void coroutine_fn throttle_group_co_io_limits_intercept(ThrottleGroupMember *tgm
typedef struct {
ThrottleGroupMember *tgm;
ThrottleDirection direction;
+ bool reset_timer_armed;
} RestartData;
static void coroutine_fn throttle_group_restart_queue_entry(void *opaque)
@@ -403,6 +404,9 @@ static void coroutine_fn throttle_group_restart_queue_entry(void *opaque)
bool empty_queue;
qemu_mutex_lock(&tg->lock);
+ if (data->reset_timer_armed) {
+ tg->any_timer_armed[direction] = false;
+ }
empty_queue = !throttle_group_co_restart_queue(tgm, direction);
/* If the request queue was empty then we have to take care of
@@ -419,18 +423,23 @@ static void coroutine_fn throttle_group_restart_queue_entry(void *opaque)
}
static void throttle_group_restart_queue(ThrottleGroupMember *tgm,
- ThrottleDirection direction)
+ ThrottleDirection direction,
+ bool reset_timer_armed)
{
Coroutine *co;
RestartData *rd = g_new0(RestartData, 1);
rd->tgm = tgm;
rd->direction = direction;
+ rd->reset_timer_armed = reset_timer_armed;
- /* This function is called when a timer is fired or when
- * throttle_group_restart_tgm() is called. Either way, there can
+ /* If reset_timer_armed is set then this means that this function
+ * was called when a timer was fired (either from timer_cb() or
+ * from throttle_group_restart_tgm()). In this case there can
* be no timer pending on this tgm at this point */
- assert(!timer_pending(tgm->throttle_timers.timers[direction]));
+ if (reset_timer_armed) {
+ assert(!timer_pending(tgm->throttle_timers.timers[direction]));
+ }
qatomic_inc(&tgm->restart_pending);
@@ -444,15 +453,50 @@ void throttle_group_restart_tgm(ThrottleGroupMember *tgm)
if (tgm->throttle_state) {
for (dir = THROTTLE_READ; dir < THROTTLE_MAX; dir++) {
- QEMUTimer *t = tgm->throttle_timers.timers[dir];
+ QEMUTimer *t;
+ ThrottleState *ts = tgm->throttle_state;
+ ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
+ bool reset_timer_armed;
+
+ /*
+ * This function restarts the tgm's queue immediately.
+ * This is used for example for callers to drain all requests.
+ * There are three different scenarios depending on whether
+ * a timer is armed for this tg and which tgm owns the timer.
+ */
+
+ qemu_mutex_lock(&tg->lock);
+
+ t = tgm->throttle_timers.timers[dir];
if (timer_pending(t)) {
- /* If there's a pending timer on this tgm, fire it now */
+ /*
+ * Case 1: this tgm has a pending timer.
+ * We can fire the timer immediately.
+ */
timer_del(t);
- timer_cb(tgm, dir);
+ reset_timer_armed = true;
+ } else if (tg->any_timer_armed[dir]) {
+ /*
+ * Case 2: another tgm has a pending timer.
+ * In this case we can still restart the queue but we
+ * have to leave any_timer_armed untouched so the
+ * other tgm's timer is not disrupted.
+ */
+ reset_timer_armed = false;
} else {
- /* Else run the next request from the queue manually */
- throttle_group_restart_queue(tgm, dir);
+ /*
+ * Case 3: there is no timer set for this group.
+ * Here we can simulate a timer that fires immediately,
+ * so the queue is restarted but no other thread
+ * can arm a timer in the meantime.
+ */
+ tg->any_timer_armed[dir] = true;
+ reset_timer_armed = true;
}
+
+ qemu_mutex_unlock(&tg->lock);
+
+ throttle_group_restart_queue(tgm, dir, reset_timer_armed);
}
}
}
@@ -499,16 +543,13 @@ void throttle_group_get_config(ThrottleGroupMember *tgm, ThrottleConfig *cfg)
*/
static void timer_cb(ThrottleGroupMember *tgm, ThrottleDirection direction)
{
- ThrottleState *ts = tgm->throttle_state;
- ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
-
- /* The timer has just been fired, so we can update the flag */
- qemu_mutex_lock(&tg->lock);
- tg->any_timer_armed[direction] = false;
- qemu_mutex_unlock(&tg->lock);
-
- /* Run the request that was waiting for this timer */
- throttle_group_restart_queue(tgm, direction);
+ /*
+ * Run the request that was waiting for this timer.
+ * tg->any_timer_armed needs to be cleared, but we'll do it later
+ * when the queue is restarted in order to prevent another thread
+ * from arming the timer before that.
+ */
+ throttle_group_restart_queue(tgm, direction, true);
}
static void read_timer_cb(void *opaque)
--
2.47.3
^ permalink raw reply related [flat|nested] 4+ messages in thread* Re: [PATCH v3 0/1] Fix race condition in throttle_group_restart_queue()
2026-03-12 12:11 [PATCH v3 0/1] Fix race condition in throttle_group_restart_queue() Alberto Garcia
2026-03-12 12:12 ` [PATCH v3 1/1] throttle-group: " Alberto Garcia
@ 2026-03-16 16:36 ` Hanna Czenczek
1 sibling, 0 replies; 4+ messages in thread
From: Hanna Czenczek @ 2026-03-16 16:36 UTC (permalink / raw)
To: Alberto Garcia, qemu-devel; +Cc: Jorge Merlino, Kevin Wolf, qemu-block
On 12.03.26 13:11, Alberto Garcia wrote:
> Hi,
>
> In the previous patch I forgot to relax the assertion for case 2.
>
> If we're restarting the queue when another tgm has a timer armed I
> don't think we can guarantee that when the timer fires it won't set a
> new timer on our own tgm before we reach the assertion.
>
> I think that the assertion should depend on reset_timer_armed:
>
> - If it's true, no one else can arm a timer so the assertion holds.
> - If it's false, it is possible for the other tgm to arm our timer.
> The assertion would fail in this case, but I think it's harmless
> because even if it would fire at the same time the logic in
> throttle_group_restart_queue_entry() is protected by the lock.
>
> Berto
>
> Alberto Garcia (1):
> throttle-group: Fix race condition in throttle_group_restart_queue()
>
> block/throttle-groups.c | 79 +++++++++++++++++++++++++++++++----------
> 1 file changed, 60 insertions(+), 19 deletions(-)
Thanks, applied to my block branch:
https://gitlab.com/hreitz/qemu/-/commits/block
Hanna
^ permalink raw reply [flat|nested] 4+ messages in thread