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Date: Thu, 20 Mar 2008 23:07:16 -0400 (EDT) From: Alan Stern <stern@...land.harvard.edu> To: Andrew Morton <akpm@...ux-foundation.org> cc: Michael Buesch <mb@...sch.de>, Henrique de Moraes Holschuh <hmh@....eng.br>, David Brownell <david-b@...bell.net>, Richard Purdie <rpurdie@...ys.net>, <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>, Geert Uytterhoeven <geert@...ux-m68k.org>, <netdev@...r.kernel.org>, Martin Schwidefsky <schwidefsky@...ibm.com>, Heiko Carstens <heiko.carstens@...ibm.com>, <linux-usb@...r.kernel.org>, <linux-wireless@...r.kernel.org>, <video4linux-list@...hat.com>, Stefan Richter <stefanr@...6.in-berlin.de>, <lm-sensors@...sensors.org> Subject: Re: use of preempt_count instead of in_atomic() at leds-gpio.c On Thu, 20 Mar 2008, Andrew Morton wrote: > > > > Now, it happens that in_atomic() returns true on non-preemtible kernels > > > > when running in interrupt or softirq context. But if the above code really > > > > is using in_atomic() to detect am-i-called-from-interrupt and NOT > > > > am-i-called-from-inside-spinlock, they should be using in_irq(), > > > > in_softirq() or in_interrupt(). > > > > > > Presumably most of these places are actually trying to detect > > > am-i-allowed-to-sleep. Isn't that what in_atomic() is supposed to do? > > > > No, I think there is no such check in the kernel. Most likely for performance > > reasons, as it would require a global flag that is set on each spinlock. > > Yup. non-preemptible kernels avoid the inc/dec of > current_thread_info->preempt_count on spin_lock/spin_unlock So then what's the point of having in_atomic() at all? Is it nothing more than a shorthand form of (in_irq() | in_softirq() | in_interrupt())? In short, you are saying that there is _no_ reliable way to determine am-i-called-from-inside-spinlock. Well, why isn't there? Would it be so terrible if non-preemptible kernels did adjust preempt_count on spin_lock/unlock? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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