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    SubjectRe: [PATCH 2/2] block: Avoid deadlocks with bio allocation by stacking drivers
    Hello, Kent.

    On Mon, Sep 10, 2012 at 02:56:33PM -0700, Kent Overstreet wrote:
    > commit df7e63cbffa3065fcc4ba2b9a93418d7c7312243
    > Author: Kent Overstreet <>
    > Date: Mon Sep 10 14:33:46 2012 -0700
    > block: Avoid deadlocks with bio allocation by stacking drivers
    > Previously, if we ever try to allocate more than once from the same bio
    > set while running under generic_make_request() (i.e. a stacking block
    > driver), we risk deadlock.
    > This is because of the code in generic_make_request() that converts
    > recursion to iteration; any bios we submit won't actually be submitted
    > (so they can complete and eventually be freed) until after we return -
    > this means if we allocate a second bio, we're blocking the first one
    > from ever being freed.
    > Thus if enough threads call into a stacking block driver at the same
    > time with bios that need multiple splits, and the bio_set's reserve gets
    > used up, we deadlock.
    > This can be worked around in the driver code - we could check if we're
    > running under generic_make_request(), then mask out __GFP_WAIT when we
    > go to allocate a bio, and if the allocation fails punt to workqueue and
    > retry the allocation.
    > But this is tricky and not a generic solution. This patch solves it for
    > all users by inverting the previously described technique. We allocate a
    > rescuer workqueue for each bio_set, and then in the allocation code if
    > there are bios on current->bio_list we would be blocking, we punt them
    > to the rescuer workqueue to be submitted.
    > This guarantees forward progress for bio allocations under
    > generic_make_request() provided each bio is submitted before allocating
    > the next, and provided the bios are freed after they complete.
    > Note that this doesn't do anything for allocation from other mempools.
    > Instead of allocating per bio data structures from a mempool, code
    > should use bio_set's front_pad.
    > Tested it by forcing the rescue codepath to be taken (by disabling the
    > first GFP_NOWAIT) attempt, and then ran it with bcache (which does a lot
    > of arbitrary bio splitting) and verified that the rescuer was being
    > invoked.
    > Signed-off-by: Kent Overstreet <>
    > CC: Jens Axboe <>

    I'm still a bit scared but think this is correct.

    Acked-by: Tejun Heo <>

    One last thing is that we may want to add @name on bioset creation so
    that we can name the workqueue properly but that's for another patch.



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