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    SubjectRe: Using RCU with rcu_read_lock()?
    On Fri, Jun 15, 2007 at 04:25:02PM -0400, Dmitry Torokhov wrote:
    > On 6/15/07, Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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.

    Thanx, Paul
    -
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