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    Subject[PATCH 5.1 309/405] rcu: Do a single rhp->func read in rcu_head_after_call_rcu()
    Date
    [ Upstream commit b699cce1604e828f19c39845252626eb78cdf38a ]

    The rcu_head_after_call_rcu() function reads the rhp->func pointer twice,
    which can result in a false-positive WARN_ON_ONCE() if the callback
    were passed to call_rcu() between the two reads. Although racing
    rcu_head_after_call_rcu() with call_rcu() is to be a dubious use case
    (the return value is not reliable in that case), intermittent and
    irreproducible warnings are also quite dubious. This commit therefore
    uses a single READ_ONCE() to pick up the value of rhp->func once, then
    tests that value twice, thus guaranteeing consistent processing within
    rcu_head_after_call_rcu()().

    Neverthless, racing rcu_head_after_call_rcu() with call_rcu() is still
    a dubious use case.

    Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
    [ paulmck: Add blank line after declaration per checkpatch.pl. ]
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    include/linux/rcupdate.h | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    index 6cdb1db776cf9..922bb68488133 100644
    --- a/include/linux/rcupdate.h
    +++ b/include/linux/rcupdate.h
    @@ -878,9 +878,11 @@ static inline void rcu_head_init(struct rcu_head *rhp)
    static inline bool
    rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
    {
    - if (READ_ONCE(rhp->func) == f)
    + rcu_callback_t func = READ_ONCE(rhp->func);
    +
    + if (func == f)
    return true;
    - WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
    + WARN_ON_ONCE(func != (rcu_callback_t)~0L);
    return false;
    }

    --
    2.20.1


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