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Date: Fri, 20 Apr 2012 15:26:13 +0800 From: Bryan Wu <bryan.wu@...onical.com> To: Stephen Boyd <sboyd@...eaurora.org> Cc: linux@....linux.org.uk, rpurdie@...ys.net, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, linus.walleij@...aro.org, akpm@...ux-foundation.org, arnd.bergmann@...aro.org, nicolas.pitre@...aro.org, tim.gardner@...onical.com Subject: Re: [PATCH 01/18] led-triggers: create a trigger for CPU activity On Fri, Apr 20, 2012 at 2:59 PM, Stephen Boyd <sboyd@...eaurora.org> wrote: > On 4/17/2012 3:53 AM, Bryan Wu wrote: >> +static int __init ledtrig_cpu_init(void) >> +{ >> + int cpu; >> + >> + /* Supports up to 9999 cpu cores */ >> + BUILD_BUG_ON(CONFIG_NR_CPUS > 9999); >> + >> + /* >> + * Registering CPU led trigger for each CPU cores here >> + * ignores CPU hotplug, but after this CPU hotplug works >> + * fine with this trigger. >> + */ >> + for_each_possible_cpu(cpu) { >> + struct led_trigger *trig; >> + char *name = per_cpu(trig_name, cpu); >> + struct rw_semaphore *lock = &per_cpu(trig_lock, cpu); >> + >> + init_rwsem(lock); > > What is this lock protecting? > >> + >> + snprintf(name, MAX_NAME_LEN, "cpu%d", cpu); >> + >> + down_write(lock); >> + led_trigger_register_simple(name, &trig); >> + per_cpu(cpu_trig, cpu) = trig; >> + up_write(lock); > > This is the only place it is locked and unlocked. > > I think the previous comment was about how led_trigger_register_simple() > exposed a live trigger but it wasn't assigned to the per_cpu variable > yet. Can we not just reorder the assignment so we have > > struct led_trigger *trig = NULL; > ... > per_cpu(cpu_trig, cpu) = trig; > led_trigger_register_simple(); > I thinked about reordering like you posted here before, but led_trigger_register_simple(const char *name, struct led_trigger **tp) will modify the value of pointer trig here, because it use **tp to do that. So a lock here just protects the critical region, looks like safer to kill race condition pointed out by Tim. Thanks, -- Bryan Wu <bryan.wu@...onical.com> Kernel Developer +86.186-168-78255 Mobile Canonical Ltd. www.canonical.com Ubuntu - Linux for human beings | www.ubuntu.com -- 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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