[PATCH] USB: gadget: f_hid: fix deadlock in f_hidg_write()

[Radoslav Gerganov]

In f_hidg_write() the write_spinlock is acquired before calling
usb_ep_queue() which causes a deadlock when dummy_hcd is being used.
This is because dummy_queue() callbacks into f_hidg_req_complete() which
tries to acquire the same spinlock. This is (part of) the backtrace when
the deadlock occurs:

  0xffffffffc06b1410 in f_hidg_req_complete
  0xffffffffc06a590a in usb_gadget_giveback_request
  0xffffffffc06cfff2 in dummy_queue
  0xffffffffc06a4b96 in usb_ep_queue
  0xffffffffc06b1eb6 in f_hidg_write
  0xffffffff8127730b in __vfs_write
  0xffffffff812774d1 in vfs_write
  0xffffffff81277725 in SYSC_write

Fix this by releasing the write_spinlock before calling usb_ep_queue()

Signed-off-by: Radoslav Gerganov <rgerga...@vmware.com>
---
 drivers/usb/gadget/function/f_hid.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/gadget/function/f_hid.c 
b/drivers/usb/gadget/function/f_hid.c
index 54e859d..492bb44 100644
--- a/drivers/usb/gadget/function/f_hid.c
+++ b/drivers/usb/gadget/function/f_hid.c
@@ -391,20 +391,20 @@ static ssize_t f_hidg_write(struct file *file, const char 
__user *buffer,
        req->complete = f_hidg_req_complete;
        req->context  = hidg;
 
+       spin_unlock_irqrestore(&hidg->write_spinlock, flags);
+
        status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
        if (status < 0) {
                ERROR(hidg->func.config->cdev,
                        "usb_ep_queue error on int endpoint %zd\n", status);
-               goto release_write_pending_unlocked;
+               goto release_write_pending;
        } else {
                status = count;
        }
-       spin_unlock_irqrestore(&hidg->write_spinlock, flags);
 
        return status;
 release_write_pending:
        spin_lock_irqsave(&hidg->write_spinlock, flags);
-release_write_pending_unlocked:
        hidg->write_pending = 0;
        spin_unlock_irqrestore(&hidg->write_spinlock, flags);
 
-- 
1.9.1