Add support for no_interrupt (#956)

The function fuse_session_process_buf_int() would do much things
for FUSE_INTERRUPT requests, even there are no FUSE_INTERRUPT requests:

1. check every non-FUSE_INTERRUPT request and add these requests to the
linked list(se->list) under a big lock(se->lock).
2. the function fuse_free_req() frees every request and remove them from
the linked list(se->list) under a bing lock(se->lock).

These operations are not meaningful when there are no FUSE_INTERRUPT requests,
and have a great impact on the performance of fuse filesystem because the big
lock for each request.

In some cases, FUSE_INTERRUPT requests are infrequent, even none at all.
Besides, the user-defined filesystem may do nothing for FUSE_INTERRUPT requests.

And the kernel side has the option "no_interrupt" in struct fuse_conn. This kernel option
can be enabled by return ENOSYS in libfuse for the reply of FUSE_INTERRUPT request.
But I don't find the code to enable the "no_interrupt" kernel option in libfuse.

So add the no_interrupt support, and when this operaion is enabled:
1. remove the useless locking operaions and list operations.
2. return ENOSYS for the reply of FUSE_INTERRUPT request to inform the kernel to disable
FUSE_INTERRUPT request.

1 files changed, 11 insertions, 1 deletions

diff --git a/include/fuse_common.h b/include/fuse_common.h
index ba4b127..c583af3 100644
--- a/include/fuse_common.h
+++ b/include/fuse_common.h
@@ -635,9 +635,19 @@ struct fuse_conn_info {
 	unsigned max_backing_stack_depth;
 
 	/**
+	 * Disable FUSE_INTERRUPT requests.
+	 *
+	 * Enable `no_interrupt` option to:
+	 * 1) Avoid unnecessary locking operations and list operations,
+	 * 2) Return ENOSYS for the reply of FUSE_INTERRUPT request to
+	 * inform the kernel not to send the FUSE_INTERRUPT request.
+	 */
+	unsigned no_interrupt;
+
+	/**
 	 * For future use.
 	 */
-	unsigned reserved[21];
+	unsigned reserved[20];
 };
 
 struct fuse_session;