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Date: Thu, 9 Feb 2017 17:12:46 +0100 (CET) From: Thomas Gleixner <tglx@...utronix.de> To: Christoph Lameter <cl@...ux.com> cc: Michal Hocko <mhocko@...nel.org>, Mel Gorman <mgorman@...hsingularity.net>, Vlastimil Babka <vbabka@...e.cz>, Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, syzkaller <syzkaller@...glegroups.com>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc On Thu, 9 Feb 2017, Christoph Lameter wrote: > On Thu, 9 Feb 2017, Thomas Gleixner wrote: > > > > The stop_machine would need to ensure that all cpus cease processing > > > before proceeding. > > > > Ok. I try again: > > > > CPU 0 CPU 1 > > for_each_online_cpu(cpu) > > ==> cpu = 1 > > stop_machine() > > > > Stops processing on all CPUs by preempting the current execution and > > forcing them into a high priority busy loop with interrupts disabled. > > Exactly that means we are outside of the sections marked with > get_online_cpous(). > > > It does exactly what you describe. It stops processing on all other cpus > > until release, but that does not invalidate any data on those cpus. > > Why would it need to invalidate any data? The change of the cpu masks > would need to be done when the machine is stopped. This sounds exactly > like what we need and much of it is already there. You are just not getting it, really. The problem is that this for_each_online_cpu() is racy against a concurrent hot unplug and therefor can queue stuff for a not longer online cpu. That's what the mm folks tried to avoid by preventing a CPU hotplug operation before entering that loop. > Lets get rid of get_online_cpus() etc. And that solves what? Can you please start to understand the scope of the whole hotplug machinery including the requirements for get_online_cpus() before you waste everybodys time with your uninformed and halfbaken proposals? Thanks, tglx
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