video: hyperv_fb: Avoid taking busy spinlock on panic path

[ Upstream commit 1d044ca035dc22df0d3b39e56f2881071d9118bd ]

The Hyper-V framebuffer code registers a panic notifier in order
to try updating its fbdev if the kernel crashed. The notifier
callback is straightforward, but it calls the vmbus_sendpacket()
routine eventually, and such function takes a spinlock for the
ring buffer operations.

Panic path runs in atomic context, with local interrupts and
preemption disabled, and all secondary CPUs shutdown. That said,
taking a spinlock might cause a lockup if a secondary CPU was
disabled with such lock taken. Fix it here by checking if the
ring buffer spinlock is busy on Hyper-V framebuffer panic notifier;
if so, bail-out avoiding the potential lockup scenario.

Cc: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Michael Kelley <mikelley@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Tianyu Lan <Tianyu.Lan@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Tested-by: Fabio A M Martins <fabiomirmar@gmail.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20220819221731.480795-10-gpiccoli@igalia.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 59a4aa8..c6692fd 100644 (file)
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -280,6 +280,19 @@ void hv_ringbuffer_cleanup(struct hv_ring_buffer_info *ring_info)
        ring_info->pkt_buffer_size = 0;
 }
 
+/*
+ * Check if the ring buffer spinlock is available to take or not; used on
+ * atomic contexts, like panic path (see the Hyper-V framebuffer driver).
+ */
+
+bool hv_ringbuffer_spinlock_busy(struct vmbus_channel *channel)
+{
+       struct hv_ring_buffer_info *rinfo = &channel->outbound;
+
+       return spin_is_locked(&rinfo->ring_lock);
+}
+EXPORT_SYMBOL_GPL(hv_ringbuffer_spinlock_busy);
+
 /* Write to the ring buffer. */
 int hv_ringbuffer_write(struct vmbus_channel *channel,
                        const struct kvec *kv_list, u32 kv_count,

diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
index 072ce07..4ff25df 100644 (file)
--- a/drivers/video/fbdev/hyperv_fb.c
+++ b/drivers/video/fbdev/hyperv_fb.c
@@ -780,12 +780,18 @@ static void hvfb_ondemand_refresh_throttle(struct hvfb_par *par,
 static int hvfb_on_panic(struct notifier_block *nb,
                         unsigned long e, void *p)
 {
+       struct hv_device *hdev;
        struct hvfb_par *par;
        struct fb_info *info;
 
        par = container_of(nb, struct hvfb_par, hvfb_panic_nb);
-       par->synchronous_fb = true;
        info = par->info;
+       hdev = device_to_hv_device(info->device);
+
+       if (hv_ringbuffer_spinlock_busy(hdev->channel))
+               return NOTIFY_DONE;
+
+       par->synchronous_fb = true;
        if (par->need_docopy)
                hvfb_docopy(par, 0, dio_fb_size);
        synthvid_update(info, 0, 0, INT_MAX, INT_MAX);

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 3b42264..646f1da 100644 (file)
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1341,6 +1341,8 @@ struct hv_ring_buffer_debug_info {
 int hv_ringbuffer_get_debuginfo(struct hv_ring_buffer_info *ring_info,
                                struct hv_ring_buffer_debug_info *debug_info);
 
+bool hv_ringbuffer_spinlock_busy(struct vmbus_channel *channel);
+
 /* Vmbus interface */
 #define vmbus_driver_register(driver)  \
        __vmbus_driver_register(driver, THIS_MODULE, KBUILD_MODNAME)