Messages in this thread | | | Date | Mon, 17 Feb 2014 09:50:49 -0500 | From | Boris Ostrovsky <> | Subject | Re: [UPDATED][PATCH v2 46/52] xen, balloon: Fix CPU hotplug callback registration |
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On 02/15/2014 11:51 AM, Srivatsa S. Bhat wrote: > On 02/14/2014 10:20 PM, Srivatsa S. Bhat wrote: >> On 02/14/2014 10:19 PM, Boris Ostrovsky wrote: >>> On 02/14/2014 02:59 AM, Srivatsa S. Bhat wrote: >>>> Subsystems that want to register CPU hotplug callbacks, as well as >>>> perform >>>> initialization for the CPUs that are already online, often do it as shown >>>> below: >>>> > [...] >>> This looks exactly like the earlier version (i.e the notifier is still >>> kept registered on allocation failure and commit message doesn't exactly >>> reflect the change). >>> >> Sorry, your earlier reply (for some unknown reason) missed the email-threading >> and landed elsewhere in my inbox, and hence unfortunately I forgot to take >> your suggestions into account while sending out the v2. >> >> I'll send out an updated version of just this patch, as a reply. > Here is the updated patch. Please let me know what you think!
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
-boris
> > ---------------------------------------------------------------------------- > > From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> > Subject: [PATCH] xen, balloon: Fix CPU hotplug callback registration > > Subsystems that want to register CPU hotplug callbacks, as well as perform > initialization for the CPUs that are already online, often do it as shown > below: > > get_online_cpus(); > > for_each_online_cpu(cpu) > init_cpu(cpu); > > register_cpu_notifier(&foobar_cpu_notifier); > > put_online_cpus(); > > This is wrong, since it is prone to ABBA deadlocks involving the > cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently > with CPU hotplug operations). > > The xen balloon driver doesn't take get/put_online_cpus() around this code, > but that is also buggy, since it can miss CPU hotplug events in between the > initialization and callback registration: > > for_each_online_cpu(cpu) > init_cpu(cpu); > ^ > | Race window; Can miss CPU hotplug events here. > v > register_cpu_notifier(&foobar_cpu_notifier); > > Interestingly, the balloon code in xen can simply be reorganized as shown > below, to have a race-free method to register hotplug callbacks, without even > taking get/put_online_cpus(). This is because the initialization performed for > already online CPUs is exactly the same as that performed for CPUs that come > online later. Moreover, the code has checks in place to avoid double > initialization. > > register_cpu_notifier(&foobar_cpu_notifier); > > get_online_cpus(); > > for_each_online_cpu(cpu) > init_cpu(cpu); > > put_online_cpus(); > > A hotplug operation that occurs between registering the notifier and calling > get_online_cpus(), won't disrupt anything, because the code takes care to > perform the memory allocations only once. > > So reorganize the balloon code in xen this way to fix the issues with CPU > hotplug callback registration. > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> > Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> > Cc: David Vrabel <david.vrabel@citrix.com> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: xen-devel@lists.xenproject.org > Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> > --- > > drivers/xen/balloon.c | 36 ++++++++++++++++++++++++------------ > 1 file changed, 24 insertions(+), 12 deletions(-) > > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c > index 37d06ea..dd79549 100644 > --- a/drivers/xen/balloon.c > +++ b/drivers/xen/balloon.c > @@ -592,19 +592,29 @@ static void __init balloon_add_region(unsigned long start_pfn, > } > } > > +static int alloc_balloon_scratch_page(int cpu) > +{ > + if (per_cpu(balloon_scratch_page, cpu) != NULL) > + return 0; > + > + per_cpu(balloon_scratch_page, cpu) = alloc_page(GFP_KERNEL); > + if (per_cpu(balloon_scratch_page, cpu) == NULL) { > + pr_warn("Failed to allocate balloon_scratch_page for cpu %d\n", cpu); > + return -ENOMEM; > + } > + > + return 0; > +} > + > + > static int balloon_cpu_notify(struct notifier_block *self, > unsigned long action, void *hcpu) > { > int cpu = (long)hcpu; > switch (action) { > case CPU_UP_PREPARE: > - if (per_cpu(balloon_scratch_page, cpu) != NULL) > - break; > - per_cpu(balloon_scratch_page, cpu) = alloc_page(GFP_KERNEL); > - if (per_cpu(balloon_scratch_page, cpu) == NULL) { > - pr_warn("Failed to allocate balloon_scratch_page for cpu %d\n", cpu); > + if (alloc_balloon_scratch_page(cpu)) > return NOTIFY_BAD; > - } > break; > default: > break; > @@ -624,15 +634,17 @@ static int __init balloon_init(void) > return -ENODEV; > > if (!xen_feature(XENFEAT_auto_translated_physmap)) { > - for_each_online_cpu(cpu) > - { > - per_cpu(balloon_scratch_page, cpu) = alloc_page(GFP_KERNEL); > - if (per_cpu(balloon_scratch_page, cpu) == NULL) { > - pr_warn("Failed to allocate balloon_scratch_page for cpu %d\n", cpu); > + register_cpu_notifier(&balloon_cpu_notifier); > + > + get_online_cpus(); > + for_each_online_cpu(cpu) { > + if (alloc_balloon_scratch_page(cpu)) { > + put_online_cpus(); > + unregister_cpu_notifier(&balloon_cpu_notifier); > return -ENOMEM; > } > } > - register_cpu_notifier(&balloon_cpu_notifier); > + put_online_cpus(); > } > > pr_info("Initialising balloon driver\n"); > > >
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