linux-kernel - Re: [PATCH 2/3] ocfs2: Remove a useless spinlock

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Date:   Tue, 19 Jul 2022 15:25:11 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     David Laight <David.Laight@...LAB.COM>,
        Mark Fasheh <mark@...heh.com>,
        Joel Becker <jlbec@...lplan.org>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>
Subject: Re: [PATCH 2/3] ocfs2: Remove a useless spinlock

Le 19/07/2022 à 12:24, David Laight a écrit :
> From: Christophe JAILLET
>> Sent: 19 July 2022 11:02
>>
>> 'node_map_lock' is a spinlock only used to protect calls to set_bit(),
>> clear_bit() and test_bit().
>>
>> {set|clear}_bit() are already atomic and don't need this extra spinlock.
>> test_bit() only reads the bitmap for a given bit.
>>
>> Remove this useless spinlock.
> 
> It looks to me like the calling code is racy
> unless there is another lock in the callers.

The call chains are:
   ocfs2_recover_orphans()
     ocfs2_mark_recovering_orphan_dir()
       spin_lock(&osb->osb_lock);		<-- osb_lock spinlock
       ocfs2_node_map_set_bit()			<-- uses node_map_lock
       ...
       spin_unlock(&osb->osb_lock);
     ...
     ocfs2_clear_recovering_orphan_dir()
       ocfs2_node_map_clear_bit()		<-- uses node_map_lock
						    osb_lock is NOT taken


   ocfs2_check_orphan_recovery_state()
     spin_lock(&osb->osb_lock);			<-- osb_lock spinlock
     ...
     ocfs2_node_map_test_bit()			<-- uses node_map_lock
     ...
     spin_unlock(&osb->osb_lock);


So the code looks already protected by the 'osb_lock' spinlock, but I 
don't know this code and ocfs2_mark_recovering_orphan_dir() looks tricky 
to me. (so some other eyes are much welcome)

> While map->map is protected, the result of test_bit()
> is stale - so can't be used for much.
> 

Anyway, should there be a locking issue, it is there with or without my 
patch, right?

CJ


> 	David
> 
>>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>> test_bit() is NOT documented as an atomic function. However, I can't see
>> how it could return a wrong result here.
>>
>> So review with care. There is maybe something I don't think about that is
>> lurking here.
>> ---
>>   fs/ocfs2/heartbeat.c | 11 ++++-------
>>   fs/ocfs2/ocfs2.h     |  2 --
>>   2 files changed, 4 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/ocfs2/heartbeat.c b/fs/ocfs2/heartbeat.c
>> index 1d72e0788943..4863ad35c242 100644
>> --- a/fs/ocfs2/heartbeat.c
>> +++ b/fs/ocfs2/heartbeat.c
>> @@ -35,7 +35,6 @@ static void ocfs2_node_map_init(struct ocfs2_node_map *map)
>>
>>   void ocfs2_init_node_maps(struct ocfs2_super *osb)
>>   {
>> -	spin_lock_init(&osb->node_map_lock);
>>   	ocfs2_node_map_init(&osb->osb_recovering_orphan_dirs);
>>   }
>>
>> @@ -67,9 +66,8 @@ void ocfs2_node_map_set_bit(struct ocfs2_super *osb,
>>   	if (bit==-1)
>>   		return;
>>   	BUG_ON(bit >= map->num_nodes);
>> -	spin_lock(&osb->node_map_lock);
>> +
>>   	set_bit(bit, map->map);
>> -	spin_unlock(&osb->node_map_lock);
>>   }
>>
>>   void ocfs2_node_map_clear_bit(struct ocfs2_super *osb,
>> @@ -79,9 +77,8 @@ void ocfs2_node_map_clear_bit(struct ocfs2_super *osb,
>>   	if (bit==-1)
>>   		return;
>>   	BUG_ON(bit >= map->num_nodes);
>> -	spin_lock(&osb->node_map_lock);
>> +
>>   	clear_bit(bit, map->map);
>> -	spin_unlock(&osb->node_map_lock);
>>   }
>>
>>   int ocfs2_node_map_test_bit(struct ocfs2_super *osb,
>> @@ -89,13 +86,13 @@ int ocfs2_node_map_test_bit(struct ocfs2_super *osb,
>>   			    int bit)
>>   {
>>   	int ret;
>> +
>>   	if (bit >= map->num_nodes) {
>>   		mlog(ML_ERROR, "bit=%d map->num_nodes=%d\n", bit, map->num_nodes);
>>   		BUG();
>>   	}
>> -	spin_lock(&osb->node_map_lock);
>> +
>>   	ret = test_bit(bit, map->map);
>> -	spin_unlock(&osb->node_map_lock);
>>   	return ret;
>>   }
>>
>> diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
>> index 740b64238312..1df193b97c30 100644
>> --- a/fs/ocfs2/ocfs2.h
>> +++ b/fs/ocfs2/ocfs2.h
>> @@ -302,8 +302,6 @@ struct ocfs2_super
>>
>>   	u32 *slot_recovery_generations;
>>
>> -	spinlock_t node_map_lock;
>> -
>>   	u64 root_blkno;
>>   	u64 system_dir_blkno;
>>   	u64 bitmap_blkno;
>> --
>> 2.34.1
> 
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