Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

From: Alexey Gladkov <gladkov.alexey@gmail.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Jens Axboe <axboe@kernel.dk>, Kees Cook <keescook@chromium.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
	Oleg Nesterov <oleg@redhat.com>, Linux-MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	io-uring <io-uring@vger.kernel.org>
Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting
Date: Thu, 21 Jan 2021 17:07:42 +0100	[thread overview]
Message-ID: <20210121160742.evd3632lepfytlxb@example.org> (raw)
In-Reply-To: <87ft2u2ss5.fsf@x220.int.ebiederm.org>

On Thu, Jan 21, 2021 at 09:50:34AM -0600, Eric W. Biederman wrote:
> >> The current ucount code does check for overflow and fails the increment
> >> in every case.
> >> 
> >> So arguably it will be a regression and inferior error handling behavior
> >> if the code switches to the ``better'' refcount_t data structure.
> >> 
> >> I originally didn't use refcount_t because silently saturating and not
> >> bothering to handle the error makes me uncomfortable.
> >> 
> >> Not having to acquire the ucounts_lock every time seems nice.  Perhaps
> >> the path forward would be to start with stupid/correct code that always
> >> takes the ucounts_lock for every increment of ucounts->count, that is
> >> later replaced with something more optimal.
> >> 
> >> Not impacting performance in the non-namespace cases and having good
> >> performance in the other cases is a fundamental requirement of merging
> >> code like this.
> >
> > Did I understand your suggestion correctly that you suggest to use
> > spin_lock for atomic_read and atomic_inc ?
> >
> > If so, then we are already incrementing the counter under ucounts_lock.
> >
> > 	...
> > 	if (atomic_read(&ucounts->count) == INT_MAX)
> > 		ucounts = NULL;
> > 	else
> > 		atomic_inc(&ucounts->count);
> > 	spin_unlock_irq(&ucounts_lock);
> > 	return ucounts;
> >
> > something like this ?
> 
> Yes.  But without atomics.  Something a bit more like:
> > 	...
> > 	if (ucounts->count == INT_MAX)
> > 		ucounts = NULL;
> > 	else
> > 		ucounts->count++;
> > 	spin_unlock_irq(&ucounts_lock);
> > 	return ucounts;

This is the original code.

> I do believe at some point we will want to say using the spin_lock for
> ucounts->count is cumbersome, and suboptimal and we want to change it to
> get a better performing implementation.
> 
> Just for getting the semantics correct we should be able to use just
> ucounts_lock for locking.  Then when everything is working we can
> profile and optimize the code.
> 
> I just don't want figuring out what is needed to get hung up over little
> details that we can change later.

OK. So I will drop this my change for now.

-- 
Rgrds, legion

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