linux-kernel - Re: [PATCH] mm/vmalloc: Fix unlock order in s

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Date:   Sun, 13 Dec 2020 14:42:51 -0500
From:   Waiman Long <longman@...hat.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmalloc: Fix unlock order in s_stop()

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@...hat.com>
>> ---
>>   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@...il.com>
>
> Thanks!
>
> --
> Vlad Rezki
>