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    SubjectRe: [PATCH v4] lib: cpu_rmap: avoid flushing all workqueues
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    On Wed, 2013-01-02 at 15:12 -0800, Andrew Morton wrote:
    > On Wed, 2 Jan 2013 13:52:25 -0800
    > David Decotigny <decot@googlers.com> wrote:
    >
    > > In some cases, free_irq_cpu_rmap() is called while holding a lock
    > > (eg. rtnl). This can lead to deadlocks, because it invokes
    > > flush_scheduled_work() which ends up waiting for whole system
    > > workqueue to flush, but some pending works might try to acquire the
    > > lock we are already holding.
    > >
    > > This commit uses reference-counting to replace
    > > irq_run_affinity_notifiers(). It also removes
    > > irq_run_affinity_notifiers() altogether.
    >
    > I can't say that I've ever noticed cpu_rmap.c before :( Is is too late
    > to review it?
    >
    > - The naming is chaotic. At least these:
    >
    > EXPORT_SYMBOL(alloc_cpu_rmap);
    > EXPORT_SYMBOL(free_cpu_rmap);
    > EXPORT_SYMBOL(cpu_rmap_add);
    > EXPORT_SYMBOL(cpu_rmap_update);
    > EXPORT_SYMBOL(free_irq_cpu_rmap);
    > EXPORT_SYMBOL(irq_cpu_rmap_add);
    >
    > should be consistently named cpu_rmap_foo()

    There is a common practice of defining alloc_foo() and free_foo()
    alongside foo_do_this() and foo_do_that(). I deliberately chose to
    follow that. If this is deprecated then it should be documented
    somewhere.

    There is also a separation between functions that are specific to IRQ
    affinity (last 2) and those that are not (first 4).

    > - What's the locking model? It appears to be caller-provided, but
    > it is undocumented.

    I think caller-provided can be assumed as the default for library code.
    And IRQ setup and teardown need to be properly serialised in the driver
    already.

    > drivers/net/ethernet/mellanox/mlx4/ appears to be using
    > msix_ctl.pool_lock for exclusion, but I didn't check for coverage.
    >
    > drivers/net/ethernet/sfc/efx.c seems to not need locking because
    > all its cpu_rmap operations are at module_init() time.
    >
    > The cpu_rmap code would be less of a hand grenade if each of its
    > interface functions documented the caller's locking requirements.

    This particular 'hand grenade' *was* documented. So I don't think
    documentation is the problem.

    > As for this patch: there's no cc:stable here but it does appear that
    > the problem is sufficiently serious to justify a backport, agree?
    [...]

    Not sure. So far as I can see, nothing called free_irq_cpu_rmap() while
    holding the RTNL lock before v3.8-rc1. If there can be work items on a
    global workqueue that lock a PCI device (perhaps EEH?) then stable
    versions may also be affected.

    Ben.

    --
    Ben Hutchings, Staff Engineer, Solarflare
    Not speaking for my employer; that's the marketing department's job.
    They asked us to note that Solarflare product names are trademarked.



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