Le 20/07/2022 à 03:59, Joseph Qi a écrit :
On 7/19/22 9:25 PM, Christophe JAILLET wrote:
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)
osb_lock is to protect osb filed such as 'osb_orphan_wipes', while
node_map_lock is to protect the node map 'osb_recovering_orphan_dirs'
specifically.
Thanks for this explanation.
But does "node_map_lock" really protects anything?
It is just around some atomic function calls which shouldn't need any,
right?
test_bit() is not documented as atomic, but {clear|set}_bit() could be
executed just before or just after it with the current locking
mechanism, so I don't really see how it would make a difference.
I don't understand the logic of this lock here.
Can you elaborate?
CJ
Thanks,
Joseph
