linux-kernel - Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount

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Message-ID: <20160509073933.GC3430@twins.programming.kicks-ass.net>
Date:	Mon, 9 May 2016 09:39:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	mingo@...nel.org, tglx@...utronix.de, Waiman.Long@....com,
	jason.low2@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount

On Sun, May 08, 2016 at 09:56:08PM -0700, Davidlohr Bueso wrote:
> Read waiters are currently reference counted from the time it enters
> the slowpath until the lock is released and the waiter is awoken. This
> is fragile and superfluous considering everything occurs within down_read()
> without returning to the caller, and the very nature of the primitive does
> not suggest that the task can disappear from underneath us. In addition,
> spurious wakeups can make the whole refcount useless as get_task_struct()
> is only called when setting up the waiter.

So I think you're wrong here; imagine this:


	rwsem_down_read_failed()			rwsem_wake()
	  get_task_struct();
	  raw_spin_lock_irq(&wait_lock);
	  list_add_tail(&waiter.list, &wait_list);
	  raw_spin_unlock_irq(&wait_lock);
							  raw_spin_lock_irqsave(&wait_lock)
							  __rwsem_do_wake()
	  while (true) {
	    set_task_state(tsk, TASK_UNINTERRUPTIBLE);
							    waiter->task = NULL
	    if (!waiter.task) // true
	      break;

	  __set_task_state(tsk, TASK_RUNNING);

	do_exit();
							    wake_up_process(tsk); /* BOOM */