• Ben Collins's avatar
    Revert "dm crypt: fix deadlock when async crypto algorithm returns -EBUSY" · 34c26c01
    Ben Collins authored
    [ Upstream commit c0403ec0 ]
    
    This reverts Linux 4.1-rc1 commit 0618764c.
    
    The problem which that commit attempts to fix actually lies in the
    Freescale CAAM crypto driver not dm-crypt.
    
    dm-crypt uses CRYPTO_TFM_REQ_MAY_BACKLOG.  This means the the crypto
    driver should internally backlog requests which arrive when the queue is
    full and process them later.  Until the crypto hw's queue becomes full,
    the driver returns -EINPROGRESS.  When the crypto hw's queue if full,
    the driver returns -EBUSY, and if CRYPTO_TFM_REQ_MAY_BACKLOG is set, is
    expected to backlog the request and process it when the hardware has
    queue space.  At the point when the driver takes the request from the
    backlog and starts processing it, it calls the completion function with
    a status of -EINPROGRESS.  The completion function is called (for a
    second time, in the case of backlogged requests) with a status/err of 0
    when a request is done.
    
    Crypto drivers for hardware without hardware queueing use the helpers,
    crypto_init_queue(), crypto_enqueue_request(), crypto_dequeue_request()
    and crypto_get_backlog() helpers to implement this behaviour correctly,
    while others implement this behaviour without these helpers (ccp, for
    example).
    
    dm-crypt (before the patch that needs reverting) uses this API
    correctly.  It queues up as many requests as the hw queues will allow
    (i.e. as long as it gets back -EINPROGRESS from the request function).
    Then, when it sees at least one backlogged request (gets -EBUSY), it
    waits till that backlogged request is handled (completion gets called
    with -EINPROGRESS), and then continues.  The references to
    af_alg_wait_for_completion() and af_alg_complete() in that commit's
    commit message are irrelevant because those functions only handle one
    request at a time, unlink dm-crypt.
    
    The problem is that the Freescale CAAM driver, which that commit
    describes as having being tested with, fails to implement the
    backlogging behaviour correctly.  In cam_jr_enqueue(), if the hardware
    queue is full, it simply returns -EBUSY without backlogging the request.
    What the observed deadlock was is not described in the commit message
    but it is obviously the wait_for_completion() in crypto_convert() where
    dm-crypto would wait for the completion being called with -EINPROGRESS
    in the case of backlogged requests.  This completion will never be
    completed due to the bug in the CAAM driver.
    
    Commit 0618764c incorrectly made dm-crypt wait for every request,
    even when the driver/hardware queues are not full, which means that
    dm-crypt will never see -EBUSY.  This means that that commit will cause
    a performance regression on all crypto drivers which implement the API
    correctly.
    
    Revert it.  Correct backlog handling should be implemented in the CAAM
    driver instead.
    
    Cc'ing stable purely because commit 0618764c did.  If for some reason
    a stable@ kernel did pick up commit 0618764c it should get reverted.
    Signed-off-by: default avatarRabin Vincent <rabin.vincent@axis.com>
    Reviewed-by: default avatarHoria Geanta <horia.geanta@freescale.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
    Signed-off-by: default avatarSasha Levin <sasha.levin@oracle.com>
    34c26c01
dm-crypt.c 47.6 KB