Re: Using RCU with rcu_read_lock()?

[Dmitry Torokhov]

On 6/15/07, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Jun 15, 2007 at 04:25:02PM -0400, Dmitry Torokhov wrote:
> > On 6/15/07, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > >On Sat, Jun 16, 2007 at 12:59:40AM +0530, Dipankar Sarma wrote:
> > >> On Fri, Jun 15, 2007 at 09:04:19PM +0200, Peter Zijlstra wrote:
> > >> > On Fri, 2007-06-15 at 15:00 -0400, Dmitry Torokhov wrote:
> > >> > > Hi,
> > >> > >
> > >> > > I have a piece of code that is always called under a spinlock with
> > >> > > interrups disabled. Within that piece of code I iterate through a
> > >> > > list. I have another piece of code that wants to modify that list. I
> > >> > > have 2 options:
> > >> > >
> > >> > > I don't want to do 1) because the otheir piece of code does not
> > >really
> > >> > > care about object owning the spinlock and so acquiring the spinlock
> > >is
> > >> > > "not nice". However it is guaranteed that the piece of code that
> > >> > > accesses lock runs atomically with interrupts disabled. So
> > >> > > rcu_read_lock() would be superfluos there.
> > >> > >
> > >> > > Is it possible to still use list_for_each_rcu() and friends to access
> > >> > > that list without rcu_read_lock()? Or it is betteruse complete RCU
> > >> > > interface and eat cost of couple of extra instrctions?
> > >> >
> > >> > Yes, preemptible rcu requires that you use the full interface, also, it
> > >> > more clearly documents the code. Trying to find code that breaks these
> > >> > assumptions is very tedious work after the fact.
> > >> >
> > >> > Please do use the RCU interface in full.
> > >>
> > >> As Peter said, you should use the strict RCU APIs and not rely
> > >> on the current implementation of RCU to optimize. Things change.
> > >> Plus static/dynamic checking becomes easier that way.
> > >
> > >What they said!!!
> > >
> > >There are a couple of other options, however:
> > >
> > >1.      Use preempt_disable() and preempt_enable() on the read side,
> > >       and synchronize_sched() on the update side.
> > >
> > >2.      Use local_irq_save() and local_irq_restore() on the read side,
> > >       and synchronize_sched() on the update side.  Usually not
> > >       competitive -- unless interrupts needed to be disabled for some
> > >       other reason anyway.  Which you in fact say that you do.
> >
> > Right. The callsite that iterates through the list is essentially
> > protected by spin_lock_irqsave()/spin_unlock_irqrestore() - needed for
> > other reasons (such as updating internal state of a device - and that
> > can happen from different contexts).
>
> That will work!
>
> > >I believe that #2 might do what you want.  But please, PLEASE carefully
> > >comment this usage!!!
> >
> > Would there be a reson not to use #2 but rather full RCU with
> > rcu_read_lock()/synchronize_rcu()?
>
> Probably not, but here are a couple of situations where the full RCU
> might be preferred:
>
> 1.      If you were relying on interrupts being disabled within an
>        interrupt handler (which they are -not- in -rt), then you would
>        either need to add some form of local_irq_save() or, as you say,
>        go to the rcu_read_lock() and synchronize_rcu() interfaces.
>
> 2.      If updates needed to use callbacks rather than synchronous waits
>        for grace periods, in other words, if you needed call_rcu()
>        instead of synchronize_rcu().  Of course, a callback API for
>        _sched (call_rcu_sched() or some such) could be added if needed,
>        though it would be better to avoid the API proliferation unless
>        really badly needed.

OK, then I will got with route #2 and just comment it properly.

Thank you for your answers.

--
Dmitry
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