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    Subject[PATCH v2 02/16] rcu/tree: Skip entry into the page allocator for PREEMPT_RT
    Date
    From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

    To keep the kfree_rcu() code working in purely atomic sections on RT,
    such as non-threaded IRQ handlers and raw spinlock sections, avoid
    calling into the page allocator which uses sleeping locks on RT.

    In fact, even if the caller is preemptible, the kfree_rcu() code is
    not, as the krcp->lock is a raw spinlock.

    Calling into the page allocator is optional and avoiding it should be
    Ok, especially with the page pre-allocation support in future patches.
    Such pre-allocation would further avoid the a need for a dynamically
    allocated page in the first place.

    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
    Co-developed-by: Uladzislau Rezki <urezki@gmail.com>
    Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
    ---
    kernel/rcu/tree.c | 12 ++++++++++++
    1 file changed, 12 insertions(+)

    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 6e967a9d6704..d28010cf158b 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -3068,6 +3068,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
    if (!bnode) {
    WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);

    + /*
    + * To keep this path working on raw non-preemptible
    + * sections, prevent the optional entry into the
    + * allocator as it uses sleeping locks. In fact, even
    + * if the caller of kfree_rcu() is preemptible, this
    + * path still is not, as krcp->lock is a raw spinlock.
    + * With additional page pre-allocation in the works,
    + * hitting this return is going to be much less likely.
    + */
    + if (IS_ENABLED(CONFIG_PREEMPT_RT))
    + return false;
    +
    bnode = (struct kfree_rcu_bulk_data *)
    __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
    }
    --
    2.20.1
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