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srcu: Explain why callbacks invocations can't run concurrently

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If an SRCU barrier is queued while callbacks are running and a new
callbacks invocator for the same sdp were to run concurrently, the
RCU barrier might execute too early. As this requirement is non-obvious,
make sure to keep a record.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
This commit is contained in:
Frederic Weisbecker 2023-10-04 01:29:03 +02:00 committed by Neeraj Upadhyay (AMD)
parent 94c55b9e21
commit c21357e446
1 changed files with 6 additions and 0 deletions
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6
kernel/rcu/srcutree.c
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@ -1715,6 +1715,11 @@ static void srcu_invoke_callbacks(struct work_struct *work)
WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
/*
* Although this function is theoretically re-entrant, concurrent
* callbacks invocation is disallowed to avoid executing an SRCU barrier
* too early.
*/
if (sdp->srcu_cblist_invoking ||
!rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
spin_unlock_irq_rcu_node(sdp);
@ -1745,6 +1750,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
sdp->srcu_cblist_invoking = false;
more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
spin_unlock_irq_rcu_node(sdp);
/* An SRCU barrier or callbacks from previous nesting work pending */
if (more)
srcu_schedule_cbs_sdp(sdp, 0);
}