Hi! On 14:35 Thu 18 Dec , luca ellero wrote: > I all, > I was reading "Unreliable Guide To Locking" by Rusty Russell: > http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/index.html > > but the table (named Cheat Sheet For Locking) is quite confusing me: > > http://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/c214.html#MINIMUM-LOCK-REQIREMENTS > > maybe it is this really _cheating_ and _unreliable_ :-( > > So I want to ask you a two questions: > > 1. following the table it seems that, if there is a critical section > only in one place in user context, no locking is needed (if you cross > "User Context A" with "User Context A" you get "None"). You need locking *only* if there may be more than one cpu/threads, which try to execute the critical section at the same time. > But as far as I > know, if you have critical section in a "read" syscall for example, you > must use some locking because two user processes can read the same > device at the same time. If 2 user-space processes try to read at the same time, you have to keep critical sections/locking in mind. If they cannot (open for the second process fails?), you do not have to lock these operations. BTW: Do *not* use semaphores any more, except if you need/use the counting mechanism. Use mutexes instead (see Documentation/mutex-design.txt) > 2. Why spin_lock_irqsave is used only between "IRQ Handler A" and "IRQ > Handler B"? In all other situation spin_lock_irq is used instead. As far > as I know you can use this function (spin_lock_irq) only if you are > absolutely sure interrupts are not already disabled. So my question is > how can we be absolutely sure in all this situation that interrupts are > not already disabled. There are 3 different ways you can handle interrupts when locking spinlocks: spin_lock: interrupts have to be *disabled* when calling this function spin_lock_irq: interrupts have to be *enabled* when calling this function and are disabled afterwards - After spin_unlock_irq, they are enabled. This is the case even if they were disabled before, which is probably not what you want. spin_lock_irqsave: may be called with interrupts enabled and disabled - The interrupt state is stored in a local "iflags" variable and is restored during spin_unlock_irqrestore. Use this, if you are not sure about the interrupt state. -Michi -- programing a layer 3+4 network protocol for mesh networks see http://michaelblizek.twilightparadox.com -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
