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srcu: Explain the reason behind the read side critical section on GP start

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Tell about the need to protect against concurrent updaters who may
overflow the GP counter behind the current update.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2022-10-13 19:22:43 +02:00 committed by Paul E. McKenney
parent 6b77bb9b99
commit ae3c070616
1 changed files with 5 additions and 0 deletions
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5
kernel/rcu/srcutree.c
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@ -1158,6 +1158,11 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
int ss_state;
check_init_srcu_struct(ssp);
/*
* While starting a new grace period, make sure we are in an
* SRCU read-side critical section so that the grace-period
* sequence number cannot wrap around in the meantime.
*/
idx = __srcu_read_lock_nmisafe(ssp, false);
ss_state = smp_load_acquire(&ssp->srcu_size_state);
if (ss_state < SRCU_SIZE_WAIT_CALL)