Re: [PATCH RFC tip/core/rcu 1/5] rcu: Reduce overhead of cond_resched() checks for RCU

[Oleg Nesterov]

On 06/23, Paul E. McKenney wrote:
>
> On Mon, Jun 23, 2014 at 06:43:21PM +0200, Oleg Nesterov wrote:
> >
> > should equally work, or ACCESS_ONCE() can't be used to RMW ?
>
> It can be, but Linus doesn't like it to be.  I recently changed all of
> the RMW ACCESS_ONCE() calls as a result.  One of the reasons for avoiding
> RMW ACCESS_ONCE() is that language features that might one day replace
> ACCESS_ONCE() do not support RMW use.

OK, thanks.

> > Or even INIT_LIST_HEAD_RCU(). The comment in list_splice_init_rcu() says:
> >
> > 	/*
> > 	 * "first" and "last" tracking list, so initialize it.  RCU readers
> > 	 * have access to this list, so we must use INIT_LIST_HEAD_RCU()
> > 	 * instead of INIT_LIST_HEAD().
> > 	 */
> >
> > 	INIT_LIST_HEAD_RCU(list);
> >
> > but we are going to call synchronize_rcu() or something similar, this should
> > act as compiler barrier too?
>
> Indeed, synchronize_rcu() enforces a barrier on each CPU between
> any prior and subsequent accesses to RCU-protected data by that CPU.
> (Which means that CPUs that would otherwise sleep through the entire
> grace period can continue sleeping, given that it is not accessing
> any RCU-protected data while sleeping.)  I would guess load-tearing
> or store-tearing concerns.

But the kernel depends on the fact that "long" should be updated atomically,
and the concurent reader should see the old-or-new value without any tricks.

Perhaps we should add ACCESS_ONCE_PARANOID_FOR_COMPILER(). Otherwise when
you read the code it is not always clear why it is uses ACCESS_ONCE(), and
sometimes this look as if you simply do not understand it. Or at least a
/* not really needed but gcc can have bugs */ could help in these cases.

Oleg.

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