switch fput to task_work_add
... and schedule_work() for interrupt/kernel_thread callers
(and yes, now it *is* OK to call from interrupt).
We are guaranteed that __fput() will be done before we return
to userland (or exit). Note that for fput() from a kernel
thread we get an async behaviour; it's almost always OK, but
sometimes you might need to have __fput() completed before
you do anything else. There are two mechanisms for that -
a general barrier (flush_delayed_fput()) and explicit
__fput_sync(). Both should be used with care (as was the
case for fput() from kernel threads all along). See comments
in fs/file_table.c for details.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/file_table.c b/fs/file_table.c
index 9ace278..b3fc4d6 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -23,6 +23,8 @@
#include <linux/lglock.h>
#include <linux/percpu_counter.h>
#include <linux/percpu.h>
+#include <linux/hardirq.h>
+#include <linux/task_work.h>
#include <linux/ima.h>
#include <linux/atomic.h>
@@ -251,7 +253,6 @@
}
fops_put(file->f_op);
put_pid(file->f_owner.pid);
- file_sb_list_del(file);
if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
i_readcount_dec(inode);
if (file->f_mode & FMODE_WRITE)
@@ -263,10 +264,77 @@
mntput(mnt);
}
+static DEFINE_SPINLOCK(delayed_fput_lock);
+static LIST_HEAD(delayed_fput_list);
+static void delayed_fput(struct work_struct *unused)
+{
+ LIST_HEAD(head);
+ spin_lock_irq(&delayed_fput_lock);
+ list_splice_init(&delayed_fput_list, &head);
+ spin_unlock_irq(&delayed_fput_lock);
+ while (!list_empty(&head)) {
+ struct file *f = list_first_entry(&head, struct file, f_u.fu_list);
+ list_del_init(&f->f_u.fu_list);
+ __fput(f);
+ }
+}
+
+static void ____fput(struct callback_head *work)
+{
+ __fput(container_of(work, struct file, f_u.fu_rcuhead));
+}
+
+/*
+ * If kernel thread really needs to have the final fput() it has done
+ * to complete, call this. The only user right now is the boot - we
+ * *do* need to make sure our writes to binaries on initramfs has
+ * not left us with opened struct file waiting for __fput() - execve()
+ * won't work without that. Please, don't add more callers without
+ * very good reasons; in particular, never call that with locks
+ * held and never call that from a thread that might need to do
+ * some work on any kind of umount.
+ */
+void flush_delayed_fput(void)
+{
+ delayed_fput(NULL);
+}
+
+static DECLARE_WORK(delayed_fput_work, delayed_fput);
+
void fput(struct file *file)
{
- if (atomic_long_dec_and_test(&file->f_count))
+ if (atomic_long_dec_and_test(&file->f_count)) {
+ struct task_struct *task = current;
+ file_sb_list_del(file);
+ if (unlikely(in_interrupt() || task->flags & PF_KTHREAD)) {
+ unsigned long flags;
+ spin_lock_irqsave(&delayed_fput_lock, flags);
+ list_add(&file->f_u.fu_list, &delayed_fput_list);
+ schedule_work(&delayed_fput_work);
+ spin_unlock_irqrestore(&delayed_fput_lock, flags);
+ return;
+ }
+ init_task_work(&file->f_u.fu_rcuhead, ____fput);
+ task_work_add(task, &file->f_u.fu_rcuhead, true);
+ }
+}
+
+/*
+ * 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.
+ */
+void __fput_sync(struct file *file)
+{
+ if (atomic_long_dec_and_test(&file->f_count)) {
+ struct task_struct *task = current;
+ file_sb_list_del(file);
+ BUG_ON(!(task->flags & PF_KTHREAD));
__fput(file);
+ }
}
EXPORT_SYMBOL(fput);