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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Tue, 25 Feb 2020 19:54:00 +0100
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.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
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> > > > 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 (!rcu_preempt_depth() && !in_atomic())
        return false;
#endif

    return true;
}
<snip>

Thanks!

--
Vlad Rezki