On 12/13/20 1:39 PM, Uladzislau Rezki wrote:
On Sun, Dec 13, 2020 at 01:08:43PM -0500, Waiman Long wrote:
When multiple locks are acquired, they should be released in reverse
order. For s_start() and s_stop() in mm/vmalloc.c, that is not the
case.
s_start: mutex_lock(&vmap_purge_lock); spin_lock(&vmap_area_lock);
s_stop : mutex_unlock(&vmap_purge_lock); spin_unlock(&vmap_area_lock);
This unlock sequence, though allowed, is not optimal. If a waiter is
present, mutex_unlock() will need to go through the slowpath of waking
up the waiter with preemption disabled. Fix that by releasing the
spinlock first before the mutex.
Fixes: e36176be1c39 ("mm/vmalloc: rework vmap_area_lock")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
mm/vmalloc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6ae491a8b210..75913f685c71 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3448,11 +3448,11 @@ static void *s_next(struct seq_file *m, void *p, loff_t *pos)
}
static void s_stop(struct seq_file *m, void *p)
- __releases(&vmap_purge_lock)
__releases(&vmap_area_lock)
+ __releases(&vmap_purge_lock)
{
- mutex_unlock(&vmap_purge_lock);
spin_unlock(&vmap_area_lock);
+ mutex_unlock(&vmap_purge_lock);
}
static void show_numa_info(struct seq_file *m, struct vm_struct *v)
BTW, if navigation over both list is an issue, for example when there
are multiple heavy readers of /proc/vmallocinfo, i think, it make sense
to implement RCU safe lists iteration and get rid of both locks.
Making it lockless is certainly better, but doing lockless the right way
is tricky. I will probably keep it as it unless there is a significant
advantage of doing so.
Cheers,
Longman
As for the patch: Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Thanks!
--
Vlad Rezki
