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