[PATCH v2] hv: hv_fcopy: drop the obsolete message on transfer failure

[Dexuan Cui <decui@microsoft.com>]

In the case the user-space daemon crashes, hangs or is killed, we
need to down the semaphore, otherwise, after the daemon starts next
time, the obsolete data in fcopy_transaction.message or
fcopy_transaction.fcopy_msg will be used immediately.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---

v2: I removed the "FCP" prefix as Greg asked.

    I also updated the output message a little:
    "FCP: failed to acquire the semaphore" --> 
    "can not acquire the semaphore: it is benign"

 drivers/hv/hv_fcopy.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/hv/hv_fcopy.c b/drivers/hv/hv_fcopy.c
index 23b2ce2..c518ad9 100644
--- a/drivers/hv/hv_fcopy.c
+++ b/drivers/hv/hv_fcopy.c
@@ -86,6 +86,15 @@ static void fcopy_work_func(struct work_struct *dummy)
 	 * process the pending transaction.
 	 */
 	fcopy_respond_to_host(HV_E_FAIL);
+
+	/* In the case the user-space daemon crashes, hangs or is killed, we
+	 * need to down the semaphore, otherwise, after the daemon starts next
+	 * time, the obsolete data in fcopy_transaction.message or
+	 * fcopy_transaction.fcopy_msg will be used immediately.
+	 */
+	if (down_trylock(&fcopy_transaction.read_sema))
+		pr_debug("can not acquire the semaphore: it is benign\n");
+
 }
 
 static int fcopy_handle_handshake(u32 version)
-- 
1.9.1