• Waiman Long's avatar
    locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED flag · b3f84e48
    Waiman Long authored
    [ Upstream commit d7d760ef ]
    
    There are use cases where a rwsem can be acquired by one task, but
    released by another task. In thess cases, optimistic spinning may need
    to be disabled.  One example will be the filesystem freeze/thaw code
    where the task that freezes the filesystem will acquire a write lock
    on a rwsem and then un-owns it before returning to userspace. Later on,
    another task will come along, acquire the ownership, thaw the filesystem
    and release the rwsem.
    
    Bit 0 of the owner field was used to designate that it is a reader
    owned rwsem. It is now repurposed to mean that the owner of the rwsem
    is not known. If only bit 0 is set, the rwsem is reader owned. If bit
    0 and other bits are set, it is writer owned with an unknown owner.
    One such value for the latter case is (-1L). So we can set owner to 1 for
    reader-owned, -1 for writer-owned. The owner is unknown in both cases.
    
    To handle transfer of rwsem ownership, the higher level code should
    set the owner field to -1 to indicate a write-locked rwsem with unknown
    owner.  Optimistic spinning will be disabled in this case.
    
    Once the higher level code figures who the new owner is, it can then
    set the owner field accordingly.
    Tested-by: default avatarAmir Goldstein <amir73il@gmail.com>
    Signed-off-by: default avatarWaiman Long <longman@redhat.com>
    Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Davidlohr Bueso <dave@stgolabs.net>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Theodore Y. Ts'o <tytso@mit.edu>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: linux-fsdevel@vger.kernel.org
    Link: http://lkml.kernel.org/r/1526420991-21213-2-git-send-email-longman@redhat.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    Signed-off-by: default avatarSasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    b3f84e48