From: "Paul E. McKenney" <paulmck@kernel.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
"Theodore Y. Ts'o" <tytso@mit.edu>,
Ext4 Developers List <linux-ext4@vger.kernel.org>,
Suraj Jitindar Singh <surajjs@amazon.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations
Date: Tue, 25 Feb 2020 14:47:45 -0800 [thread overview]
Message-ID: <20200225224745.GX2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200225185400.GA27919@pc636>
On Tue, Feb 25, 2020 at 07:54:00PM +0100, Uladzislau Rezki wrote:
> > > > > I was thinking a 2 fold approach (just thinking out loud..):
> > > > >
> > > > > If kfree_call_rcu() is called in atomic context or in any rcu reader, then
> > > > > use GFP_ATOMIC to grow an rcu_head wrapper on the atomic memory pool and
> > > > > queue that.
> > > > >
> > > I am not sure if that is acceptable, i mean what to do when GFP_ATOMIC
> > > gets failed in atomic context? Or we can just consider it as out of
> > > memory and another variant is to say that headless object can be called
> > > from preemptible context only.
> >
> > Yes that makes sense, and we can always put disclaimer in the API's comments
> > saying if this object is expected to be freed a lot, then don't use the
> > headless-API to be extra safe.
> >
> Agree.
>
> > BTW, GFP_ATOMIC the documentation says if GFP_ATOMIC reserves are depleted,
> > the kernel can even panic some times, so if GFP_ATOMIC allocation fails, then
> > there seems to be bigger problems in the system any way. I would say let us
> > write a patch to allocate there and see what the -mm guys think.
> >
> OK. It might be that they can offer something if they do not like our
> approach. I will try to compose something and send the patch to see.
> The tree.c implementation is almost done, whereas tiny one is on hold.
>
> I think we should support batching as well as bulk interface there.
> Another way is to workaround head-less object, just to attach the head
> dynamically using kmalloc() and then call_rcu() but then it will not be
> a fair headless support :)
>
> What is your view?
>
> > > > > Otherwise, grow an rcu_head on the stack of kfree_call_rcu() and call
> > > > > synchronize_rcu() inline with it.
> > > > >
> > > > >
> > > What do you mean here, Joel? "grow an rcu_head on the stack"?
> >
> > By "grow on the stack", use the compiler-allocated rcu_head on the
> > kfree_rcu() caller's stack.
> >
> > I meant here to say, if we are not in atomic context, then we use regular
> > GFP_KERNEL allocation, and if that fails, then we just use the stack's
> > rcu_head and call synchronize_rcu() or even synchronize_rcu_expedited since
> > the allocation failure would mean the need for RCU to free some memory is
> > probably great.
> >
> Ah, i got it. I thought you meant something like recursion and then
> unwinding the stack back somehow :)
>
> > > > > Use preemptible() andr task_struct's rcu_read_lock_nesting to differentiate
> > > > > between the 2 cases.
> > > > >
> > > If the current context is preemptable then we can inline synchronize_rcu()
> > > together with freeing to handle such corner case, i mean when we are run
> > > out of memory.
> >
> > Ah yes, exactly what I mean.
> >
> OK.
>
> > > As for "task_struct's rcu_read_lock_nesting". Will it be enough just
> > > have a look at preempt_count of current process? If we have for example
> > > nested rcu_read_locks:
> > >
> > > <snip>
> > > rcu_read_lock()
> > > rcu_read_lock()
> > > rcu_read_lock()
> > > <snip>
> > >
> > > the counter would be 3.
> >
> > No, because preempt_count is not incremented during rcu_read_lock(). RCU
> > reader sections can be preempted, they just cannot goto sleep in a reader
> > section (unless the kernel is RT).
> >
> So in CONFIG_PREEMPT kernel we can identify if we are in atomic or not by
> using rcu_preempt_depth() and in_atomic(). When it comes to !CONFIG_PREEMPT
> then we skip it and consider as atomic. Something like:
>
> <snip>
> static bool is_current_in_atomic()
> {
> #ifdef CONFIG_PREEMPT_RCU
If possible: if (IS_ENABLED(CONFIG_PREEMPT_RCU))
Much nicer than #ifdef, and I -think- it should work in this case.
Thanx, Paul
> if (!rcu_preempt_depth() && !in_atomic())
> return false;
> #endif
>
> return true;
> }
> <snip>
>
> Thanks!
>
> --
> Vlad Rezki
next prev parent reply other threads:[~2020-02-25 22:47 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200215233817.GA670792@mit.edu>
[not found] ` <20200216121246.GG2935@paulmck-ThinkPad-P72>
[not found] ` <20200217160827.GA5685@pc636>
[not found] ` <20200217193314.GA12604@mit.edu>
2020-02-18 17:08 ` [PATCH RFC] ext4: fix potential race between online resizing and write operations Uladzislau Rezki
2020-02-20 4:52 ` Theodore Y. Ts'o
2020-02-21 0:30 ` Paul E. McKenney
2020-02-21 13:14 ` Uladzislau Rezki
2020-02-21 20:22 ` Paul E. McKenney
2020-02-22 22:24 ` Joel Fernandes
2020-02-23 1:10 ` Paul E. McKenney
2020-02-24 17:40 ` Uladzislau Rezki
2020-02-25 2:07 ` Joel Fernandes
2020-02-25 3:55 ` Paul E. McKenney
2020-02-25 14:17 ` Joel Fernandes
2020-02-25 16:38 ` Paul E. McKenney
2020-02-25 17:00 ` Joel Fernandes
2020-02-25 18:54 ` Uladzislau Rezki
2020-02-25 22:47 ` Paul E. McKenney [this message]
2020-02-26 13:04 ` Uladzislau Rezki
2020-02-26 15:06 ` Paul E. McKenney
2020-02-26 15:53 ` Uladzislau Rezki
2020-02-27 14:08 ` Joel Fernandes
2020-03-01 11:13 ` Uladzislau Rezki
2020-02-27 13:37 ` Joel Fernandes
2020-03-01 11:08 ` Uladzislau Rezki
2020-03-01 12:07 ` Uladzislau Rezki
2020-02-25 2:11 ` Joel Fernandes
2020-02-21 12:06 ` Joel Fernandes
2020-02-21 13:28 ` Joel Fernandes
2020-02-21 19:21 ` Uladzislau Rezki
2020-02-21 19:25 ` Uladzislau Rezki
2020-02-22 22:12 ` Joel Fernandes
2020-02-24 17:02 ` Uladzislau Rezki
2020-02-24 23:14 ` Paul E. McKenney
2020-02-25 1:48 ` Joel Fernandes
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