[PATCH V3 09/11] video/hyperv_fb: Avoid taking busy spinlock on panic path

["Guilherme G. Piccoli" <gpiccoli@igalia.com>]

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>

---

V3:
- simplified the code based on Michael's suggestion - thanks!

V2:
- new patch, based on the discussion in [0].
[0] https://lore.kernel.org/lkml/2787b476-6366-1c83-db80-0393da417497@igalia.com/


 drivers/hv/ring_buffer.c        | 13 +++++++++++++
 drivers/video/fbdev/hyperv_fb.c |  8 +++++++-
 include/linux/hyperv.h          |  2 ++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 59a4aa86d1f3..c6692fd5ab15 100644
--- 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 886c564787f1..e1b65a01fb96 100644
--- a/drivers/video/fbdev/hyperv_fb.c
+++ b/drivers/video/fbdev/hyperv_fb.c
@@ -783,12 +783,18 @@ 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 3b42264333ef..646f1da9f27e 100644
--- 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)
-- 
2.37.2