get_cpu_var() disables preemption and returns the per-CPU version of the
variable. Disabling preemption is useful to ensure atomic access to the
variable within the critical section.
In this case however, after the per-CPU version of the variable is
obtained the ->free_lock is acquired. For that reason it seems the raw
accessor could be used. It only seems that ->slots_ret should be
retested (because with disabled preemption this variable can not be set
to NULL otherwise).
This popped up during PREEMPT-RT testing because it tries to take
spinlocks in a preempt disabled section.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
On 2017-06-23 12:34:23 [+0200], Michal Hocko wrote:
> The changelog doesn't explain, why does this change matter. Disabling
> preemption shortly before taking a spinlock shouldn't make much
> difference. I suspect you care because of RT, right? In that case spell
> that in the changelog and explain why it matters.
yes, it is bad for RT. I added the RT pieces as explanation.
> Other than hat the patch looks good to me.
Thank you. +akpm.
mm/swap_slots.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 58f6c78f1dad..51c304477482 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -272,11 +272,11 @@ int free_swap_slot(swp_entry_t entry)
{
struct swap_slots_cache *cache;
- cache = &get_cpu_var(swp_slots);
+ cache = raw_cpu_ptr(&swp_slots);
if (use_swap_slot_cache && cache->slots_ret) {
spin_lock_irq(&cache->free_lock);
/* Swap slots cache may be deactivated before acquiring lock */
- if (!use_swap_slot_cache) {
+ if (!use_swap_slot_cache || !cache->slots_ret) {
spin_unlock_irq(&cache->free_lock);
goto direct_free;
}
@@ -296,7 +296,6 @@ int free_swap_slot(swp_entry_t entry)
direct_free:
swapcache_free_entries(&entry, 1);
}
- put_cpu_var(swp_slots);
return 0;
}
--
2.13.1
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