Messages in this thread |  | | Date | Thu, 06 Feb 2014 16:38:01 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH 00/51] CPU hotplug: Fix issues with callback registration |
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On 02/06/2014 04:34 PM, Srivatsa S. Bhat wrote: > On 02/06/2014 03:08 PM, Gautham R Shenoy wrote: >> Hi, >> >> On Thu, Feb 06, 2014 at 03:34:36AM +0530, Srivatsa S. Bhat wrote: >>> Hi, >>> [...] >> Couple of comments: >> >> Right now, cpu_add_remove_lock is being used to >> 1) Serialize the cpu-hotplug writers. >> >> 2) Serialize accesses to cpu_present/possible_map. >> >> 3) Serialize updates to the cpu_chain (the cpu hotplug notifier chain). >> >> - This is necessary to ensure that registration of notifiers and >> invocation of CPU_POST_DEAD notifications don't race with each >> other. Else we could have used get/put_online_cpus() in >> register_cpu_notifier() and this patch series wouldn't have been >> necessary. >> >> 4) Bulk cpu-hotplug (disable/enable_non_boot_cpus), but this is a >> special case of 1). >> >> CPU_POST_DEAD notification, is invoked with the cpu_hotplug.lock >> dropped. This was necessary for subsystems which would be waiting for >> some other thread to finish some work, and that other thread could >> invoke get_online_cpus(). If CPU_POST_DEAD notification were issued >> without dropping the cpu_hotplug.lock, this would lead to a deadlock >> as the notifier would be left stuck waiting for the thread which is >> blocked in get_online_cpus(). >> >> It was introduced to ensure that multithreaded workqueues can safely >> use get_online_cpus() [https://lkml.org/lkml/2008/6/29/121]. >> >> As of now, only two subsystems use this notification and workqueues is >> _not_ one of them! >> * arch/x86/kernel/cpu/mcheck/mce.c:mce_cpu_callback() >> * drivers/cpufreq/cpufreq.c:cpufreq_cpu_callback() >> I haven't yet audited these two cases to see if they really need this >> to be handled in CPU_POST_DEAD or if they can be handled in CPU_DEAD. >> > > Well, cpufreq had a legitimate need to use POST_DEAD to avoid the > deadlock described in commit 1aee40ac9c. However, there had been some > discussion some time ago about reorganizing the cpufreq's hotplug callback > so as to move most (but not all) of its work outside of POST_DEAD [1].
Forgot to give the link.. Here it is: http://article.gmane.org/gmane.linux.kernel/1571276
Regards, Srivatsa S. Bhat
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