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rcu: Upgrade header comment for poll_state_synchronize_rcu()

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This commit emphasizes the possibility of concurrent calls to
synchronize_rcu() and synchronize_rcu_expedited() causing one or
the other of the two grace periods being lost from the viewpoint of
poll_state_synchronize_rcu().

If you cannot afford to lose grace periods this way, you should
instead use the _full() variants of the polled RCU API, for
example, poll_state_synchronize_rcu_full().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-11-25 08:43:10 -08:00
parent 253cbbff62
commit 95ff24ee7b
1 changed files with 9 additions and 1 deletions
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10
kernel/rcu/tree.c
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@ -3559,7 +3559,9 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full);
* If @false is returned, it is the caller's responsibility to invoke this
* function later on until it does return @true. Alternatively, the caller
* can explicitly wait for a grace period, for example, by passing @oldstate
* to cond_synchronize_rcu() or by directly invoking synchronize_rcu().
* to either cond_synchronize_rcu() or cond_synchronize_rcu_expedited()
* on the one hand or by directly invoking either synchronize_rcu() or
* synchronize_rcu_expedited() on the other.
*
* Yes, this function does not take counter wrap into account.
* But counter wrap is harmless. If the counter wraps, we have waited for
@ -3570,6 +3572,12 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_full);
* completed. Alternatively, they can use get_completed_synchronize_rcu()
* to get a guaranteed-completed grace-period state.
*
* In addition, because oldstate compresses the grace-period state for
* both normal and expedited grace periods into a single unsigned long,
* it can miss a grace period when synchronize_rcu() runs concurrently
* with synchronize_rcu_expedited(). If this is unacceptable, please
* instead use the _full() variant of these polling APIs.
*
* This function provides the same memory-ordering guarantees that
* would be provided by a synchronize_rcu() that was invoked at the call
* to the function that provided @oldstate, and that returned at the end