sunrpc: allow svc threads to fail initialisation cleanly

If an svc thread needs to perform some initialisation that might fail,
it has no good way to handle the failure.

Before the thread can exit it must call svc_exit_thread(), but that
requires the service mutex to be held.  The thread cannot simply take
the mutex as that could deadlock if there is a concurrent attempt to
shut down all threads (which is unlikely, but not impossible).

nfsd currently call svc_exit_thread() unprotected in the unlikely event
that unshare_fs_struct() fails.

We can clean this up by introducing svc_thread_init_status() by which an
svc thread can report whether initialisation has succeeded.  If it has,
it continues normally into the action loop.  If it has not,
svc_thread_init_status() immediately aborts the thread.
svc_start_kthread() waits for either of these to happen, and calls
svc_exit_thread() (under the mutex) if the thread aborted.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 3 insertions, 6 deletions

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index b1dc3404173b..3fb6c8c9a2f0 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -873,11 +873,9 @@ nfsd(void *vrqstp)
 
 	/* At this point, the thread shares current->fs
 	 * with the init process. We need to create files with the
-	 * umask as defined by the client instead of init's umask. */
-	if (unshare_fs_struct() < 0) {
-		printk("Unable to start nfsd thread: out of memory\n");
-		goto out;
-	}
+	 * umask as defined by the client instead of init's umask.
+	 */
+	svc_thread_init_status(rqstp, unshare_fs_struct());
 
 	current->fs->umask = 0;
 
@@ -899,7 +897,6 @@ nfsd(void *vrqstp)
 
 	atomic_dec(&nfsd_th_cnt);
 
-out:
 	/* Release the thread */
 	svc_exit_thread(rqstp);
 	return 0;