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SubjectRace condition between iget_locked() and evict_inodes()
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Hi Al,

I think there is a race condition between iget_locked() and evict_inodes().

evict_inodes() checks i_count and if zero proceeds to take i_lock then set I_FREEING and eventually disposes of the inode.

But a concurrent iget_locked() takes i_lock and then increments i_count.

Thus if the events happen in this order:

evict_inodes() iget_locked() in find_inode_fast()
atomic_read(&inode->i_count) -> 0 take i_lock
wait on i_lock __iget(inode); -> i_count now 1
set I_FREEING drop i_lock
evict() return inode to caller

The inode is now gone due to the evict() call whilst it is happily being used with an elevated i_count by the iget_locked() calling process. It seems to me like evict_inodes() should be checking i_count inside i_lock either as the only check or it should at least re-check it.

Please tell me what I am missing here. I assume there must be something providing exclusion and I am just too blind to see it but I thought it worth bringing to your attention in case it really is simply broken.

Best regards,

Anton
--
Anton Altaparmakov <anton at tuxera.com> (replace at with @)
Lead in File System Development, Tuxera Inc., http://www.tuxera.com/
Linux NTFS maintainer

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