From: Sungho Bae <[email protected]> With no_console_suspend enabled, hvc console output can continue while virtio_console is freezing. In that window, put_chars can still enqueue buffers to the output virtqueue while virtcons_freeze is tearing queues down, triggering a BUG_ON in virtqueue_detach_unused_buf_split:
BUG_ON(vq->vq.num_free != vq->split.vring.num) Add a pm_freezing flag to ports_device. Set it via smp_store_release() at the start of virtcons_freeze(); put_chars() and __send_to_port() drop output while the flag is set, checked via smp_load_acquire(). The check in __send_to_port() is placed under outvq_lock, making it atomic with remove_port_data() which also acquires outvq_lock. Once remove_port_data() returns for a given port, no concurrent __send_to_port() can add buffers before remove_vqs() tears down the vq. After setting pm_freezing, acquire and release outvq_lock for each port before calling virtio_reset_device(). A TX thread that already passed the pm_freezing check may still hold outvq_lock while spinning for host acknowledgment (the nonblock=false hvc path); the drain loop ensures all such threads have completed before the device is reset. Clear pm_freezing in virtcons_restore() only after all port->out_vq pointers have been reassigned to the newly allocated virtqueues, preventing TX paths from dereferencing freed vqs during restore. Also fix two races in __send_to_port() uncovered by this work: load port->portdev via READ_ONCE() and check for NULL to guard against concurrent hot-unplug, and move out_vq = port->out_vq inside outvq_lock after the pm_freezing check to avoid a stale pointer. Link: https://sashiko.dev/#/patchset/20260515225259.1054-1-baver.bae%40gmail.com Signed-off-by: Sungho Bae <[email protected]> --- drivers/char/virtio_console.c | 80 ++++++++++++++++++++++++++++++++++- 1 file changed, 78 insertions(+), 2 deletions(-) diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c index 9a33217c68d9..fad673f733a8 100644 --- a/drivers/char/virtio_console.c +++ b/drivers/char/virtio_console.c @@ -157,6 +157,12 @@ struct ports_device { /* Major number for this device. Ports will be created as minors. */ int chr_major; + + /* + * Set to true during PM freeze to block TX paths that may race + * with virtqueue teardown (e.g. hvc put_chars with no_console_suspend). + */ + bool pm_freezing; }; struct port_stats { @@ -601,11 +607,30 @@ static ssize_t __send_to_port(struct port *port, struct scatterlist *sg, int err; unsigned long flags; unsigned int len; - - out_vq = port->out_vq; + struct ports_device *portdev; spin_lock_irqsave(&port->outvq_lock, flags); + portdev = READ_ONCE(port->portdev); + + if (!portdev) { + in_count = 0; + goto done; + } + + /* + * Check freeze flag under the lock so that the flag check and + * virtqueue_add_outbuf() are atomic with respect to + * remove_port_data() which also takes outvq_lock. This + * guarantees that once remove_port_data() returns, no new + * buffers can be added before remove_vqs() tears down the vq. + * Pairs with smp_store_release() in virtcons_freeze/restore. + */ + if (smp_load_acquire(&portdev->pm_freezing)) /* pairs with freeze/restore */ + goto done; + + out_vq = port->out_vq; + reclaim_consumed_buffers(port); err = virtqueue_add_outbuf(out_vq, sg, nents, data, GFP_ATOMIC); @@ -1110,11 +1135,36 @@ static ssize_t put_chars(u32 vtermno, const u8 *buf, size_t count) struct scatterlist sg[1]; void *data; int ret; + struct ports_device *portdev; port = find_port_by_vtermno(vtermno); if (!port) return -EPIPE; + /* + * Silently drop output in two cases, both by returning count so + * that the hvc layer does not spin-retry: + * + * 1. Device hot-unplug (!portdev): portdev was NULLed by + * unplug_port() after hvc_remove() was already called, so + * the hvc layer will stop invoking put_chars() very soon. + * Returning count avoids a pointless retry loop in the + * interim. + * + * 2. PM freeze (pm_freezing): the hvc console stays active + * under no_console_suspend but virtqueues are being torn + * down. Drop the output silently so the hvc layer does not + * stall suspend. + * + * This early check avoids a pointless GFP_ATOMIC allocation; + * __send_to_port() rechecks under outvq_lock for correctness. + * Pairs with smp_store_release() in virtcons_freeze/restore. + */ + portdev = READ_ONCE(port->portdev); + if (!portdev || + smp_load_acquire(&portdev->pm_freezing)) /* pairs with freeze/restore */ + return count; + data = kmemdup(buf, count, GFP_ATOMIC); if (!data) return -ENOMEM; @@ -1972,6 +2022,7 @@ static int virtcons_probe(struct virtio_device *vdev) /* Attach this portdev to this virtio_device, and vice-versa. */ portdev->vdev = vdev; vdev->priv = portdev; + portdev->pm_freezing = false; portdev->chr_major = register_chrdev(0, "virtio-portsdev", &portdev_fops); @@ -2092,6 +2143,24 @@ static int virtcons_freeze(struct virtio_device *vdev) portdev = vdev->priv; + /* + * Block TX paths (put_chars, __send_to_port) before resetting the + * device and tearing down virtqueues. This prevents races with + * hvc console writes that remain active under no_console_suspend. + */ + smp_store_release(&portdev->pm_freezing, true); + + /* + * Synchronize with any concurrent __send_to_port() that may have + * passed the pm_freezing check. By acquiring and releasing the + * outvq_lock for each port, we ensure all active TX paths have + * completed before we reset the device. + */ + list_for_each_entry(port, &portdev->ports, list) { + spin_lock_irq(&port->outvq_lock); + spin_unlock_irq(&port->outvq_lock); + } + virtio_reset_device(vdev); if (use_multiport(portdev)) @@ -2153,6 +2222,13 @@ static int virtcons_restore(struct virtio_device *vdev) if (port->guest_connected) send_control_msg(port, VIRTIO_CONSOLE_PORT_OPEN, 1); } + + /* + * Allow TX paths only after all port->out_vq pointers have + * been reassigned to the newly allocated virtqueues. + */ + smp_store_release(&portdev->pm_freezing, false); + return 0; } #endif -- 2.43.0
