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srcutiny: Make a Tiny SRCU grace period imply an RCU grace period

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In non-preemptible kernels, a Tiny SRCU grace period implies an RCU
grace period because any context switch suffices.  But in preemptible
kernels, it is possible for a Tiny SRCU grace period to elapse without
a corresponding RCU grace period.

Which was OK until RCU Tasks Trace was re-implemented in terms
of SRCU-fast, which in Tiny SRCU is implemented as SRCU, which is
already fast.  And RCU Tasks Trace grace periods are required to imply
RCU grace periods.  This commit therefore adds a synchronize_rcu(),
but only in preemptible kernels.

Because preemptible Tiny SRCU is not in mainline, this added call to
synchronize_rcu() will not slow anything down:  The comparison would
instead be with TREE SRCU.  But if this added call ever becomes a problem,
the Tiny SRCU srcu_struct structure could track whether or not this is for
SRCU-fast, and to add the synchronize_rcu() only in the SRCU-fast case.
However, at the moment, this is seen as unnecessary complexity.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2026-07-09 09:36:34 -07:00
parent 0464f587c6
commit 835a10eeb0
1 changed files with 2 additions and 0 deletions
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2
kernel/rcu/srcutiny.c
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@ -159,6 +159,8 @@ void srcu_drive_gp(struct work_struct *wp)
WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
WRITE_ONCE(ssp->srcu_gp_waiting, true); /* srcu_read_unlock() wakes! */
preempt_enable();
if (IS_ENABLED(CONFIG_PREEMPTION))
synchronize_rcu(); // Needed for RCU Tasks Trace to imply RCU grace period
do {
// Deadlock issues prevent __srcu_read_unlock() from
// doing an unconditional wakeup, so polling is required.