Messages in this thread | | | Date | Fri, 15 Jun 2007 16:25:02 -0400 | From | "Dmitry Torokhov" <> | Subject | Re: Using RCU with rcu_read_lock()? |
| |
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).
> > 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()?
Thank you.
-- Dmitry - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |