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)
Cc: Dexuan Cui
Cc: Haiyang Zhang
Cc: "K. Y. Srinivasan"
Cc: Michael Kelley
Cc: Stephen Hemminger
Cc: Tianyu Lan
Cc: Wei Liu
Tested-by: Fabio A M Martins
Signed-off-by: Guilherme G. Piccoli
---
V2:
- new patch, based on the discussion in [0].
[0]
https://lore.kernel.org/lkml/2787b476-6366-1c83-db80-0393da417...@igalia.com/
drivers/hv/ring_buffer.c| 16
drivers/video/fbdev/hyperv_fb.c | 8 +++-
include/linux/hyperv.h | 2 ++
3 files changed, 25 insertions(+), 1 deletion(-)
diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 59a4aa86d1f3..9ceb3a7e8d19 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -280,6 +280,22 @@ 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 = >outbound;
+
+ if (spin_is_locked(>ring_lock))
+ return true;
+
+ return false;
+}
+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)
--
2.37.1
___
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