Messages in this thread | | | Date | Sat, 18 Dec 2004 04:40:22 +0000 | From | Matthew Wilcox <> | Subject | Re: [KJ] Re: [PATCH] lcd: replace cli()/sti() with spin_lock_irqsave()/spin_unlock_irqrestore() |
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On Fri, Dec 17, 2004 at 10:02:16PM -0500, Jim Nelson wrote: > Jan Dittmer wrote: > >James Nelson wrote: > > > >>Remove the cli()/sti() calls in drivers/char/lcd.c > > > > > >Why is this cli() there in the first place? ioctl is already > >called under lock_kernel. > > First - a warning. Newbie on the loose, running around, asking for a whack > with the cluebat.
OK. Quick lesson (for both of you actually; Jan's response wasn't accurate).
Once upon a time, we did not support SMP. We still had to worry about races. Interrupts could arrive and touch hardware/variables that we were in the middle of poking. So if you disable interrupts (the instructions are annoying named after the senseless x86 instructions) around the critical section, you ensure that you can't be interrupted.
Then we introduced the Big Kernel Lock (aka lock_kernel). As long as all code that touched the same hardware/variables had the BKL, cli() was still safe. We were evil and redefined the cli() function to disable interrupts on *all* processors, not just the local one.
Thankfully, the ability to disable interrupts globally has been removed now. Instead, you must disable _local_ interrupts and then acquire a lock. This is spin_lock_irq()/spin_lock_irqsave(). In order to protect against an interrupt running on a *different* CPU, you have to take the same lock in the interrupt handler. This is safe because a process running on a different CPU will eventually release its lock.
So you still need to disable interrupts, even if you're running under the BKL -- interrupt routines can't acquire the BKL (because it's magic) and acquiring the BKL doesn't disable interrupts.
There's some special tricks you can use so you don't have to grab a lock, but these are advanced magic (eg RCU, seqlocks, atomics), but most drivers won't use advanced techniques; they're mostly for specialised parts of the kernel. In particular drivers shouldn't try to invent their own locking scheme, they invariably get it subtly wrong.
Hope that clears things up for you a bit; feel free to ask about the bits that I didn't explain sufficiently.
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