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Message-ID: <20150907122709.GA31811@redhat.com>
Date: Mon, 7 Sep 2015 14:27:09 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Al Viro <viro@...IV.linux.org.uk>,
Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@...il.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>,
Maciej Żenczykowski <maze@...gle.com>
Subject: [PATCH?] fput: don't abuse task_work_add() too much
On 09/05, Al Viro wrote:
>
> On Sat, Aug 29, 2015 at 02:49:21PM +0200, Oleg Nesterov wrote:
>
> > Let's look keyctl_session_to_parent(). It does task_work_cancel()
> > but only because we can not trust user-space. Otherwise we could
> > remove it and just do task_work_add(), but this needs fifo.
> >
> > Fifo just looks more sane to me.
>
> Not if it costs us.
OK, Eric reports this trivial lockless loop takes 275ms. Apparently the
list of ____fput() works is huge. Now, how much does it cost to _create_
this huge list using task_work_add() which does cmpxchg() ? I guess a bit
more. This is what should be fixed if we think this hurts performance-wise.
And again, ignoring the latency problem due to the lack of cond_resched,
I am wondering if these 275ms are actually noticable compared to the time
the next loop needs to call all these ____fput's.
> As far as files closing is concerned, the order
> really doesn't matter. Ditto for final mntput() uses of that stuff.
Sure, fput/mntput do not care about the ordering. And more, they do
not even really need task_work_add(). We can just remove it from fput()
and everything will work fine. Correctness-wise, I mean.
And yes, unfortunately we do not have in-kernel users which already
rely on fifo.
But task_work_add() is the generic API, loosing the ordering makes
it less useful or at least less convenient.
Just for (yes sure, artificial) example. Suppose we want to implement
sys_change_process_flags(int pid, uint set, uint clr). Only current
can change its ->flags, so we can use task_work_add() to do this. But
obviously only if it is fifo.
fput() differs because it does not care which process actually does
__fput(). And thus imo we should not count this user if we need to
decide do we need fifo or not.
> IMO the obvious solution is to lose the reordering...
Oh, I disagree. But I guess I can't convince you/Eric/Linus, so I have
to shut up.
Damn. But I can't relax ;) Al, Linus, could you comment the patch below?
Not for inclusion, lacks the changelog/testing, fput() can be simplified.
But as you can see it is simple. With this patch task_work_add(____fput)
will be called only once by (say) do_exit() path. ->fput_list does not
need any serialization / atomic ops / etc. Probably we also need to move
cond_resched() from task_work_run() to ____fput() after this patch.
Again, it is not that I think this actually makes sense, but since you
dislike these 275ms...
What do you think?
Oleg.
---
diff --git a/fs/file_table.c b/fs/file_table.c
index 294174d..36af701 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -241,7 +241,13 @@ static void delayed_fput(struct work_struct *unused)
static void ____fput(struct callback_head *work)
{
- __fput(container_of(work, struct file, f_u.fu_rcuhead));
+ struct task_struct *task = current;
+ do {
+ struct file *file = task->fput_list;
+ task->fput_list = file->f_u.fu_next;
+ __fput(file);
+
+ } while (task->fput_list);
}
/*
@@ -267,9 +273,19 @@ void fput(struct file *file)
struct task_struct *task = current;
if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
+ if (task->fput_list) {
+ /* task_work_add() below was already called */
+ file->f_u.fu_next = task->fput_list;
+ task->fput_list = file;
+ return;
+ }
+
init_task_work(&file->f_u.fu_rcuhead, ____fput);
- if (!task_work_add(task, &file->f_u.fu_rcuhead, true))
+ if (!task_work_add(task, &file->f_u.fu_rcuhead, true)) {
+ file->f_u.fu_next = NULL;
+ task->fput_list = file;
return;
+ }
/*
* After this task has run exit_task_work(),
* task_work_add() will fail. Fall through to delayed
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0774487..73fe16c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -830,6 +830,7 @@ struct file {
union {
struct llist_node fu_llist;
struct rcu_head fu_rcuhead;
+ struct file *fu_next;
} f_u;
struct path f_path;
struct inode *f_inode; /* cached value */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f192cfe..6f704ff 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1477,6 +1477,7 @@ struct task_struct {
struct fs_struct *fs;
/* open file information */
struct files_struct *files;
+ struct file *fput_list;
/* namespaces */
struct nsproxy *nsproxy;
/* signal handlers */
diff --git a/kernel/fork.c b/kernel/fork.c
index 03c1eaa..77c0a50 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1007,6 +1007,7 @@ static int copy_files(unsigned long clone_flags, struct task_struct *tsk)
struct files_struct *oldf, *newf;
int error = 0;
+ tsk->fput_list = NULL;
/*
* A background process may not have any files ...
*/
--
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