[PATCH] fput: Allow calling __fput_sync() from !PF_KTHREAD thread.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

__fput_sync() was introduced by commit 4a9d4b024a3102fc ("switch fput to
task_work_add") with BUG_ON(!(current->flags & PF_KTHREAD)) check, and
the only user of __fput_sync() was introduced by commit 17c0a5aaffa63da6
("make acct_kill() wait for file closing."). However, the latter commit is
effectively calling __fput_sync() from !PF_KTHREAD thread because of
schedule_work() call followed by immediate wait_for_completion() call.
That is, there is no need to defer close_work() to a WQ context. I guess
that the reason to defer was nothing but to bypass this BUG_ON() check.
While we need to remain careful about calling __fput_sync(), we can remove
bypassable BUG_ON() check from __fput_sync().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
Al, is this change acceptable?

Eric is trying to use fput()/flush_delayed_fput()/task_work_run() from
blob_to_mnt() which is going to be introduced by
https://lkml.kernel.org/r/20200702164140.4468-8-ebiederm@xmission.com
in order to make sure that a file (which was opened for writing and is
intended to be execve()d shortly) is closed by current thread before
leaving blob_to_mnt().

But since current thread might fail to find the interested file (which was
opened for writing and is intended to be execve()d shortly) and/or might find
uninterested files (which current thread does not need to process) when
multiple threads concurrently called flush_delayed_fput(), I think that we
should use __fput_sync() in order to make sure that only the interested file
is closed by current thread.

Therefore, I propose this change.

 fs/file_table.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index 656647f9575a..7c4125179469 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -359,20 +359,15 @@ void fput(struct file *file)
 }
 
 /*
- * synchronous analog of fput(); for kernel threads that might be needed
- * in some umount() (and thus can't use flush_delayed_fput() without
- * risking deadlocks), need to wait for completion of __fput() and know
- * for this specific struct file it won't involve anything that would
- * need them.  Use only if you really need it - at the very least,
- * don't blindly convert fput() by kernel thread to that.
+ * synchronous analog of fput(); for threads that need to wait for completion
+ * of __fput() and know for this specific struct file it won't involve anything
+ * that would need them.  Use only if you really need it - at the very least,
+ * don't blindly convert fput() to __fput_sync().
  */
 void __fput_sync(struct file *file)
 {
-	if (atomic_long_dec_and_test(&file->f_count)) {
-		struct task_struct *task = current;
-		BUG_ON(!(task->flags & PF_KTHREAD));
+	if (atomic_long_dec_and_test(&file->f_count))
 		__fput(file);
-	}
 }
 
 EXPORT_SYMBOL(fput);
-- 
2.18.4