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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@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Mateusz Guzik 2017-10-03 12:58:15 +02:00 committed by Al Viro
parent 5297908270
commit c02b1a9b41
2 changed files with 7 additions and 8 deletions
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4
Documentation/filesystems/porting
View File

@ -501,10 +501,6 @@ in your dentry operations instead.
is non-NULL. Note that link body isn't available anymore, so if you need it,
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

11
fs/file.c
View File

@ -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();