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    SubjectRe: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers


    On Mon, 3 Sep 2012, Kent Overstreet wrote:

    > On Mon, Sep 03, 2012 at 04:41:37PM -0400, Mikulas Patocka wrote:
    > > ... or another possibility - start a timer when something is put to
    > > current->bio_list and use that timer to pop entries off current->bio_list
    > > and submit them to a workqueue. The timer can be cpu-local so only
    > > interrupt masking is required to synchronize against the timer.
    > >
    > > This would normally run just like the current kernel and in case of
    > > deadlock, the timer would kick in and resolve the deadlock.
    >
    > Ugh. That's a _terrible_ idea.
    >
    > Remember the old plugging code? You ever have to debug performance
    > issues caused by it?

    Yes, I do remember it (and I fixed one bug that resulted in missed unplug
    and degraded performance).

    But currently, deadlocks due to exhausted mempools and bios being stalled
    in current->bio_list don't happen (or do happen below so rarely that they
    aren't reported).

    If we add a timer, it will turn a deadlock into an i/o delay, but it can't
    make things any worse.

    BTW. can these new-style timerless plugs introduce deadlocks too? What
    happens when some bios are indefinitely delayed because their requests are
    held in a plug and a mempool runs out?

    > > > I could be convinced, but right now I prefer my solution.
    > >
    > > It fixes bio allocation problem, but not other similar mempool problems in
    > > dm and md.
    >
    > I looked a bit more, and actually I think the rest of the problem is
    > pretty limited in scope - most of those mempool allocations are per
    > request, not per split.
    >
    > I'm willing to put some time into converting dm/md over to bioset's
    > front_pad. I'm having to learn the code for the immutable biovec work,
    > anyways.

    Currently, dm targets allocate request-specific data from target-specific
    mempool. mempools are in dm-crypt, dm-delay, dm-mirror, dm-snapshot,
    dm-thin, dm-verity. You can change it to allocate request-specific data
    with the bio.

    Mikulas


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