Messages in this thread | | | Date | Fri, 15 Jun 2007 17:09:43 -0400 | From | "Dmitry Torokhov" <> | Subject | Re: Using RCU with rcu_read_lock()? |
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On 6/15/07, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > 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. >
OK, then I will got with route #2 and just comment it properly.
Thank you for your answers.
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