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.
As for the patch: Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Thanks!
--
Vlad Rezki
