linux-kernel - [PATCH 2/2] vfs: grab the lock instead of blocking in __fd

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Date:   Tue,  3 Oct 2017 12:58:15 +0200
From:   Mateusz Guzik <mguzik@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     mszeredi@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] vfs: grab the lock instead of blocking in __fd_install during resizing

Explicit locking in the fallback case provides a safe state of the
table. Getting rid of blocking semantics makes __fd_install usable
again in non-sleepable contexts, which easies backporting efforts.

There is a side effect of slightly nicer assembly for the common case
as might_sleep can now be removed.

Signed-off-by: Mateusz Guzik <mguzik@...hat.com>
---
 Documentation/filesystems/porting |  4 ----
 fs/file.c                         | 11 +++++++----
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 93e0a24..17bb4dc 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -502,10 +502,6 @@ in your dentry operations instead.
 	store it as cookie.
 --
 [mandatory]
-	__fd_install() & fd_install() can now sleep. Callers should not
-	hold a spinlock	or other resources that do not allow a schedule.
---
-[mandatory]
 	any symlink that might use page_follow_link_light/page_put_link() must
 	have inode_nohighmem(inode) called before anything might start playing with
 	its pagecache.  No highmem pages should end up in the pagecache of such
diff --git a/fs/file.c b/fs/file.c
index 9d047bd..4115503 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -592,13 +592,16 @@ void __fd_install(struct files_struct *files, unsigned int fd,
 {
 	struct fdtable *fdt;
 
-	might_sleep();
 	rcu_read_lock_sched();
 
-	while (unlikely(files->resize_in_progress)) {
+	if (unlikely(files->resize_in_progress)) {
 		rcu_read_unlock_sched();
-		wait_event(files->resize_wait, !files->resize_in_progress);
-		rcu_read_lock_sched();
+		spin_lock(&files->file_lock);
+		fdt = files_fdtable(files);
+		BUG_ON(fdt->fd[fd] != NULL);
+		rcu_assign_pointer(fdt->fd[fd], file);
+		spin_unlock(&files->file_lock);
+		return;
 	}
 	/* coupled with smp_wmb() in expand_fdtable() */
 	smp_rmb();
-- 
1.8.3.1