"Gautham R. Shenoy" <e...@linux.vnet.ibm.com> writes:
> From: "Gautham R. Shenoy" <e...@linux.vnet.ibm.com>
>
> 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.
This implies we have never tested a memory hotplug that resized the HPT.
Is that really true? Or did something change?
> 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
Do we have the full stack trace?
> 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.ku...@linux.ibm.com>
> Signed-off-by: Gautham R. Shenoy <e...@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
I realise you're just copying that comment, but it seems wrong. "user
context" means userspace. I think it means "process context" doesn't it?
Also "should" should be "must" :)
> + * 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();
cheers
