• Larry Chen's avatar
    ocfs2: ocfs2_inode_lock_tracker does not distinguish lock level · 133b81f2
    Larry Chen authored
    ocfs2_inode_lock_tracker as a variant of ocfs2_inode_lock, is used to
    prevent deadlock due to recursive lock acquisition.
    
    But this function does not distinguish whether the requested level is EX
    or PR.
    
    If a RP lock has been attained, this function will immediately return
    success afterwards even an EX lock is requested.
    
    But actually the return value does not mean that the process got a EX
    lock, because ocfs2_inode_lock has not been called.
    
    When taking lock levels into account, we face some different situations:
    
    1. no lock is held
       In this case, just lock the inode and return 0
    
    2. We are holding a lock
       For this situation, things diverges into several cases
    
       wanted     holding	     what to do
       ex		ex	    see 2.1 below
       ex		pr	    see 2.2 below
       pr		ex	    see 2.1 below
       pr		pr	    see 2.1 below
    
       2.1 lock level that is been held is compatible
       with the wanted level, so no lock action will be tacken.
    
       2.2 Otherwise, an upgrade is needed, but it is forbidden.
    
    Reason why upgrade within a process is forbidden is that lock upgrade
    may cause dead lock.  The following illustrate how it happens.
    
            process 1                             process 2
    ocfs2_inode_lock_tracker(ex=0)
                                   <======   ocfs2_inode_lock_tracker(ex=1)
    
    ocfs2_inode_lock_tracker(ex=1)
    
    For the status quo of ocfs2, without this patch, neither a bug nor
    end-user impact will be caused because the wrong logic is avoided.
    
    But I'm afraid this generic interface, may be called by other developers
    in future and used in this situation.
    
      a process
    ocfs2_inode_lock_tracker(ex=0)
    ocfs2_inode_lock_tracker(ex=1)
    
    Link: http://lkml.kernel.org/r/20180510053230.17217-1-lchen@suse.comSigned-off-by: default avatarLarry Chen <lchen@suse.com>
    Reviewed-by: default avatarGang He <ghe@suse.com>
    Cc: Mark Fasheh <mark@fasheh.com>
    Cc: Joel Becker <jlbec@evilplan.org>
    Cc: Junxiao Bi <junxiao.bi@oracle.com>
    Cc: Joseph Qi <jiangqi903@gmail.com>
    Cc: Changwei Ge <ge.changwei@h3c.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    133b81f2
dlmglue.h 7.4 KB