Re: [RESEND PATCH] powerpc/pseries: Fix cpu_hotplug_lock acquisition in resize_hpt

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

On Tue, May 14, 2019 at 05:02:16PM +1000, Michael Ellerman wrote:
> "Gautham R. Shenoy" <ego@linux.vnet.ibm.com> writes:
> > From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
> >
> > Subject: Re: [RESEND PATCH] powerpc/pseries: Fix cpu_hotplug_lock acquisition in resize_hpt
> 
> ps. A "RESEND" implies the patch is unchanged and you're just resending
> it because it was ignored.
> 
> In this case it should have just been "PATCH v2", with a note below the "---"
> saying "v2: Rebased onto powerpc/next ..."

Ok. I will send a v3 :-)

> 
> cheers
> 
> > During a memory hotplug operations involving resizing of the HPT, we
> > invoke a stop_machine() to perform the resizing. In this code path, we
> > end up recursively taking the cpu_hotplug_lock, first in
> > memory_hotplug_begin() and then subsequently in stop_machine(). This
> > causes the system to hang. With lockdep enabled we get the following
> > error message before the hang.
> >
> >   swapper/0/1 is trying to acquire lock:
> >   (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: stop_machine+0x2c/0x60
> >
> >   but task is already holding lock:
> >   (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: mem_hotplug_begin+0x20/0x50
> >
> >   other info that might help us debug this:
> >    Possible unsafe locking scenario:
> >
> >          CPU0
> >          ----
> >     lock(cpu_hotplug_lock.rw_sem);
> >     lock(cpu_hotplug_lock.rw_sem);
> >
> >    *** DEADLOCK ***
> >
> > Fix this issue by
> >   1) Requiring all the calls to pseries_lpar_resize_hpt() be made
> >      with cpu_hotplug_lock held.
> >
> >   2) In pseries_lpar_resize_hpt() invoke stop_machine_cpuslocked()
> >      as a consequence of 1)
> >
> >   3) To satisfy 1), in hpt_order_set(), call mmu_hash_ops.resize_hpt()
> >      with cpu_hotplug_lock held.
> >
> > Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> > Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> > ---
> >
> > Rebased this one against powerpc/next instead of linux/master.
> >
> >  arch/powerpc/mm/book3s64/hash_utils.c | 9 ++++++++-
> >  arch/powerpc/platforms/pseries/lpar.c | 8 ++++++--
> >  2 files changed, 14 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> > index 919a861..d07fcafd 100644
> > --- a/arch/powerpc/mm/book3s64/hash_utils.c
> > +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> > @@ -38,6 +38,7 @@
> >  #include <linux/libfdt.h>
> >  #include <linux/pkeys.h>
> >  #include <linux/hugetlb.h>
> > +#include <linux/cpu.h>
> >  
> >  #include <asm/debugfs.h>
> >  #include <asm/processor.h>
> > @@ -1928,10 +1929,16 @@ static int hpt_order_get(void *data, u64 *val)
> >  
> >  static int hpt_order_set(void *data, u64 val)
> >  {
> > +	int ret;
> > +
> >  	if (!mmu_hash_ops.resize_hpt)
> >  		return -ENODEV;
> >  
> > -	return mmu_hash_ops.resize_hpt(val);
> > +	cpus_read_lock();
> > +	ret = mmu_hash_ops.resize_hpt(val);
> > +	cpus_read_unlock();
> > +
> > +	return ret;
> >  }
> >  
> >  DEFINE_DEBUGFS_ATTRIBUTE(fops_hpt_order, hpt_order_get, hpt_order_set, "%llu\n");
> > diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
> > index 1034ef1..2fc9756 100644
> > --- a/arch/powerpc/platforms/pseries/lpar.c
> > +++ b/arch/powerpc/platforms/pseries/lpar.c
> > @@ -859,7 +859,10 @@ static int pseries_lpar_resize_hpt_commit(void *data)
> >  	return 0;
> >  }
> >  
> > -/* Must be called in user context */
> > +/*
> > + * Must be called in user context. The caller should hold the
> > + * cpus_lock.
> > + */
> >  static int pseries_lpar_resize_hpt(unsigned long shift)
> >  {
> >  	struct hpt_resize_state state = {
> > @@ -913,7 +916,8 @@ static int pseries_lpar_resize_hpt(unsigned long shift)
> >  
> >  	t1 = ktime_get();
> >  
> > -	rc = stop_machine(pseries_lpar_resize_hpt_commit, &state, NULL);
> > +	rc = stop_machine_cpuslocked(pseries_lpar_resize_hpt_commit,
> > +				     &state, NULL);
> >  
> >  	t2 = ktime_get();
> >  
> > -- 
> > 1.9.4
>