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rcu-tasks: Document that RCU Tasks Trace grace periods now imply RCU grace periods

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Now that RCU Tasks Trace is implemented in terms of SRCU-fast, the fact
that each SRCU-fast grace period implies at least two RCU grace periods
in turn means that each RCU Tasks Trace grace period implies at least
two grace periods.  This commit therefore updates the documentation
accordingly.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
This commit is contained in:
Paul E. McKenney
2026-01-14 16:18:30 -08:00
committed by Joel Fernandes
parent 4968907016
commit ad6ef775cb
2 changed files with 10 additions and 6 deletions
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7
Documentation/RCU/Design/Requirements/Requirements.rst
View File

@@ -2787,6 +2787,13 @@ which avoids the read-side memory barriers, at least for architectures
that apply noinstr to kernel entry/exit code (or that build with
``CONFIG_TASKS_TRACE_RCU_NO_MB=y``.
Now that the implementation is based on SRCU-fast, a call
to synchronize_rcu_tasks_trace() implies at least one call to
synchronize_rcu(), that is, every Tasks Trace RCU grace period contains
at least one plain vanilla RCU grace period. Should there ever
be a synchronize_rcu_tasks_trace_expedited(), this guarantee would
*not* necessarily apply to this hypothetical API member.
The tasks-trace-RCU API is also reasonably compact,
consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(),
rcu_read_lock_trace_held(), call_rcu_tasks_trace(),

9
include/linux/rcupdate.h
View File

@@ -208,12 +208,9 @@ static inline void exit_tasks_rcu_finish(void) { }
/**
* rcu_trace_implies_rcu_gp - does an RCU Tasks Trace grace period imply an RCU grace period?
*
* As an accident of implementation, an RCU Tasks Trace grace period also
* acts as an RCU grace period. However, this could change at any time.
* Code relying on this accident must call this function to verify that
* this accident is still happening.
*
* You have been warned!
* Now that RCU Tasks Trace is implemented in terms of SRCU-fast, a
* call to synchronize_rcu_tasks_trace() is guaranteed to imply at least
* one call to synchronize_rcu().
*/
static inline bool rcu_trace_implies_rcu_gp(void) { return true; }