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Subject[PATCH 2/2] vfs: grab the lock instead of blocking in __fd_install during resizing
Date
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@redhat.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
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