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    SubjectRe: [PATCH tip/core/rcu 1/6] documentation: Document call_rcu() safety mechanisms and limitations
    On Mon, Feb 17, 2014 at 01:26:48PM -0800, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    >
    > The call_rcu() family of primitives will take action to accelerate
    > grace periods when the number of callbacks pending on a given CPU
    > becomes excessive. Although this safety mechanism can be useful,
    > it is no substitute for users of call_rcu() having rate-limit controls
    > in place. This commit adds this nuance to the documentation.
    >
    > Reported-by: "Michael S. Tsirkin" <mst@redhat.com>
    > Reported-by: Gleb Natapov <gleb@redhat.com>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Grammatical nit below; otherwise:
    Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    > Documentation/RCU/checklist.txt | 19 ++++++++++++++-----
    > 1 file changed, 14 insertions(+), 5 deletions(-)
    >
    > diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
    > index 91266193b8f4..5733e31836b5 100644
    > --- a/Documentation/RCU/checklist.txt
    > +++ b/Documentation/RCU/checklist.txt
    > @@ -256,10 +256,11 @@ over a rather long period of time, but improvements are always welcome!
    > variations on this theme.
    >
    > b. Limiting update rate. For example, if updates occur only
    > - once per hour, then no explicit rate limiting is required,
    > - unless your system is already badly broken. The dcache
    > - subsystem takes this approach -- updates are guarded
    > - by a global lock, limiting their rate.
    > + once per hour, then no explicit rate limiting is
    > + required, unless your system is already badly broken.
    > + Older versions of the dcache subsystem takes this
    > + approach -- updates were guarded by a global lock,
    > + limiting their rate.

    s/takes/take/ to match the change from the singular "The dcache
    subsystem" to the plural "Older versions of the dcache subsystem"

    (You might also change " -- updates are guarded by" to ", guarding
    updates with".)

    >
    > c. Trusted update -- if updates can only be done manually by
    > superuser or some other trusted user, then it might not
    > @@ -268,7 +269,8 @@ over a rather long period of time, but improvements are always welcome!
    > the machine.
    >
    > d. Use call_rcu_bh() rather than call_rcu(), in order to take
    > - advantage of call_rcu_bh()'s faster grace periods.
    > + advantage of call_rcu_bh()'s faster grace periods. (This
    > + is only a partial solution, though.)
    >
    > e. Periodically invoke synchronize_rcu(), permitting a limited
    > number of updates per grace period.
    > @@ -276,6 +278,13 @@ over a rather long period of time, but improvements are always welcome!
    > The same cautions apply to call_rcu_bh(), call_rcu_sched(),
    > call_srcu(), and kfree_rcu().
    >
    > + Note that although these primitives do take action to avoid memory
    > + exhaustion when any given CPU has too many callbacks, a determined
    > + user could still exhaust memory. This is especially the case
    > + if a system with a large number of CPUs has been configured to
    > + offload all of its RCU callbacks onto a single CPU, or if the
    > + system has relatively little free memory.
    > +
    > 9. All RCU list-traversal primitives, which include
    > rcu_dereference(), list_for_each_entry_rcu(), and
    > list_for_each_safe_rcu(), must be either within an RCU read-side
    > --
    > 1.8.1.5
    >


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