[PATCH v2 13/18] mshv: Fix sleeping under spinlock in mshv_portid_alloc

[Stanislav Kinsburskii]

idr_alloc() is called with GFP_KERNEL inside idr_lock(), which holds a
spinlock. GFP_KERNEL allows the allocator to sleep, triggering a
sleeping-while-atomic bug.

Fix by using idr_preload(GFP_KERNEL) before taking the lock to
pre-allocate memory in a sleepable context, then idr_alloc() with
GFP_NOWAIT inside the spinlock-protected section.

Fixes: 621191d709b1 ("Drivers: hv: Introduce mshv_root module to expose 
/dev/mshv to VMMs")
Signed-off-by: Stanislav Kinsburskii <skinsburs...@linux.microsoft.com>
---
 drivers/hv/mshv_portid_table.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/mshv_portid_table.c b/drivers/hv/mshv_portid_table.c
index f1aaef69eb9b7..d6884c601b298 100644
--- a/drivers/hv/mshv_portid_table.c
+++ b/drivers/hv/mshv_portid_table.c
@@ -40,12 +40,14 @@ mshv_port_table_fini(void)
 int
 mshv_portid_alloc(struct port_table_info *info)
 {
-       int ret = 0;
+       int ret;
 
+       idr_preload(GFP_KERNEL);
        idr_lock(&port_table_idr);
        ret = idr_alloc(&port_table_idr, info, PORTID_MIN,
-                       PORTID_MAX, GFP_KERNEL);
+                       PORTID_MAX, GFP_NOWAIT);
        idr_unlock(&port_table_idr);
+       idr_preload_end();
 
        return ret;
 }