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Date: Thu, 7 Mar 2024 11:12:28 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>, linke li <lilinke99@qq.com>,
 joel@joelfernandes.org, boqun.feng@gmail.com, dave@stgolabs.net,
 frederic@kernel.org, jiangshanlai@gmail.com, josh@joshtriplett.org,
 linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com,
 qiang.zhang1211@gmail.com, quic_neeraju@quicinc.com, rcu@vger.kernel.org
Subject: Re: [PATCH] rcutorture: Fix
 rcu_torture_pipe_update_one()/rcu_torture_writer() data race and
 concurrency bug
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On Thu, 7 Mar 2024 08:20:59 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> When a write happens, it looks to see if the smallest watermark is hit,
> if so, calls irqwork to wakeup all the waiters.
> 
> The waiters will wake up, check to see if a signal is pending or if the
> ring buffer has hit the watermark the waiter was waiting for and exit the
> wait loop.
> 
> What the wait_index does, is just a way to force all waiters out of the wait
> loop regardless of the watermark the waiter is waiting for. Before a waiter
> goes into the wait loop, it saves the current wait_index. The waker will
> increment the wait_index and then call the same irq_work to wake up all the
> waiters.
> 
> After the wakeup happens, the waiter will test if the new wait_index
> matches what it was before it entered the loop, and if it is different, it
> falls out of the loop. Then the caller of the ring_buffer_wait() can
> re-evaluate if it needs to enter the wait again.
> 
> The wait_index loop exit was needed for when the file descriptor of a file
> that uses a ring buffer closes and it needs to wake up all the readers of
> that file descriptor to notify their tasks that the file closed.
> 
> So we can switch the:
> 
> 	rbwork->wait_index++;
> 	smp_wmb();
> 
> into just a:
> 
> 	(void)atomic_inc_return_release(&rbwork->wait_index);
> 
> and the:
> 
> 	smp_rmb()
> 	if (wait_index != work->wait_index)
> 
> into:
> 
> 	if (wait_index != atomic_read_acquire(&rb->wait_index))
> 
> I'll write up a patch.
> 
> Hmm, I have the same wait_index logic at the higher level doing basically
> the same thing (at the close of the file). I'll switch that over too.

Discussing this with Maitheu on IRC, we found two bugs with the current
implementation. One was a stupid bug with an easy fix, and the other is
actually a design flaw.

The first bug was the (wait_index != work->wait_index) check was done
*after* the schedule() call and not before it.

The second more fundamental bug is that there's still a race between the
first read of wait_index and the call to prepare_to_wait().

The ring_buffer code doesn't have enough context to know enough to loop or
not. If a file is being closed when another thread is just entering this
code, it could miss the wakeup.

As the callers of ring_buffer_wait() also do a loop, it's redundant to have
ring_buffer_wait() do a loop. It should just do a single wait, and then
exit and let the callers decide if it should loop again.

This will get rid of the need for the rbwork->wait_index and simplifies the
code.

Working on that patch now.

-- Steve