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    Subject[PATCH 00/11] sysv mqueue: do not hold the ipc lock unnecessarily
    This patchset continues the work that began in the sysv ipc semaphore scaling 

    Just like semaphores used to be, sysv shared memory and msg queues also abuse the ipc
    lock, unnecessarily holding it for operations such as permission and security checks. This
    patchset mostly deals with mqueues, and while shared mem can be done in a very similar way,
    I want to get these patches out in the open first. It also does some pending cleanups,
    mostly focused on the two level locking we have in ipc code, taking care of ipc_addid()
    and ipcctl_pre_down_nolock() - yes there are still functions that need to be updated as well.

    I have tried to split each patch to be as readable and specific as possible, specially when
    shortening the critical regions, going one function at a time.

    Patch 1 moves the locking to be explicitly done by the callers of ipc_addid.
    It updates msg, sem and shm.

    Patches 2-3: are just wrappers around the ipc lock, initially suggested by Linus.

    Patch 4 is similar to patch 1, moving the rcu and rw_mutex locking out of
    ipcctl_pre_down_nolock so that the callers explicitly deals with them. It updates msg, sem
    and shm.

    Patch 5 shortens the critical region in msgctl_down(), using the lockless
    ipcctl_pre_down() function and only acquiring the ipc lock for RMID and SET commands.

    Patch 6 simply moves the what-should-be lockless logic of *_INFO and *_STAT commands
    out of msgctl() into a new function, msgctl_lockless().

    Patch 7 introduces the necessary wrappers around ipc_obtain_object[_check]()
    that will later enable us to separately lookup the ipc object without holding the lock.

    Patch 8 updates the previously added msgctl_nolock() to actually be lockless, reducing
    the critical region for the STAT commands.

    Patch 9 redoes the locking for msgsend().

    Patch 10 redoes the locking for msgrcv().

    Patch 11 removes the now unused msg_lock and msg_lock_check functions, replaced by
    a smarter combination of rcu, ipc_obtain_object and ipc_object_lock.

    Davidlohr Bueso (11):
    ipc: move rcu lock out of ipc_addid
    ipc: introduce ipc object locking helpers
    ipc: close open coded spin lock calls
    ipc: move locking out of ipcctl_pre_down_nolock
    ipc,msg: shorten critical region in semctl_down
    ipc,msg: introduce msgctl_nolock
    ipc,msg: introduce lockless functions to obtain the ipc object
    ipc,msg: make msgctl_nolock lockless
    ipc,msg: reduce critical region in msgsnd
    ipc,msg: make shorten critical region in msgrcv
    ipc: remove unused functions

    ipc/msg.c | 227 ++++++++++++++++++++++++++++++++++++++-----------------------
    ipc/sem.c | 42 +++++++-----
    ipc/shm.c | 32 ++++++---
    ipc/util.c | 25 ++-----
    ipc/util.h | 22 ++++--
    5 files changed, 211 insertions(+), 137 deletions(-)



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