Messages in this thread | | | Date | Fri, 15 Jun 2007 13:12:38 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Using RCU with rcu_read_lock()? |
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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.
I believe that #2 might do what you want. But please, PLEASE carefully comment this usage!!!
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