Messages in this thread | | | From | Denis Vlasenko <> | Subject | Re: kmalloc without GFP_xxx? | Date | Wed, 29 Jun 2005 17:14:32 +0300 |
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On Wednesday 29 June 2005 16:44, Steven Rostedt wrote: > > On Wed, 29 Jun 2005, Arjan van de Ven wrote: > > > > > > but it sets irqs_disabled() IIRC. > > > > only spin_lock_irq() and co do. > > not the simple spin_lock() > > > > It may be dangerous to use spin_lock with interrupts enabled, since you > have to make sure that no interrupt ever grabs that lock. Although I do > recall seeing a few locks like this. But even so, you can transfer the > latency of the interrupts going off while holding that lock to another CPU > which IMHO is a bad thing. Also a simple spin_lock would disable
This is why I always use _irqsave. Less error prone. And locking is a very easy to get 'slightly' wrong, thus I trade 0.1% of performance for code simplicity.
> preemption with CONFIG_PREEMPT set and that would make in_atomic fail. > But to implement a kmalloc_auto you would always need to have a preempt > count. > > I'm not for a kmalloc_auto, but something like it would be useful for a > function that can work for either context, and just fail nicely if the > ATOMIC is set and the malloc can't get memory. A function like this would > currently have to always use ATOMIC even if it could have used KERNEL for > some scenarios, since it would suffer the same pitfalls as a kmalloc_auto > in determining its context.
This is more or less what I meant. Why think about each kmalloc and when you eventually did get it right: "Aha, we _sometimes_ get called from spinlocked code, GFP_ATOMIC then" - you still do atomic alloc even if cases when you were _not_ called from locked code! Thus you needed to think longer and got code which is worse. -- vda
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