From: "Paul E. McKenney" <paul...@kernel.org> There is a need for a polling interface for SRCU grace periods. This polling needs to distinguish between an SRCU instance being idle on the one hand or in the middle of a grace period on the other. This commit therefore converts the Tiny SRCU srcu_struct structure's srcu_idx from a defacto boolean to a free-running counter, using the bottom bit to indicate that a grace period is in progress. The second-from-bottom bit is thus used as the index returned by srcu_read_lock().
Link: https://lore.kernel.org/rcu/20201112201547.gf3365...@moria.home.lan/ Reported-by: Kent Overstreet <kent.overstr...@gmail.com> Signed-off-by: Paul E. McKenney <paul...@kernel.org> --- include/linux/srcutiny.h | 4 ++-- kernel/rcu/srcutiny.c | 5 +++-- 2 files changed, 5 insertions(+), 4 deletions(-) diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h index 5a5a194..fed4a2d 100644 --- a/include/linux/srcutiny.h +++ b/include/linux/srcutiny.h @@ -15,7 +15,7 @@ struct srcu_struct { short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */ - short srcu_idx; /* Current reader array element. */ + unsigned short srcu_idx; /* Current reader array element in bit 0x2. */ u8 srcu_gp_running; /* GP workqueue running? */ u8 srcu_gp_waiting; /* GP waiting for readers? */ struct swait_queue_head srcu_wq; @@ -59,7 +59,7 @@ static inline int __srcu_read_lock(struct srcu_struct *ssp) { int idx; - idx = READ_ONCE(ssp->srcu_idx); + idx = (READ_ONCE(ssp->srcu_idx) & 0x2) / 2; WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1); return idx; } diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c index 6208c1d..5598cf6 100644 --- a/kernel/rcu/srcutiny.c +++ b/kernel/rcu/srcutiny.c @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *wp) ssp->srcu_cb_head = NULL; ssp->srcu_cb_tail = &ssp->srcu_cb_head; local_irq_enable(); - idx = ssp->srcu_idx; - WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx); + idx = (ssp->srcu_idx & 0x2) / 2; + WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); WRITE_ONCE(ssp->srcu_gp_waiting, true); /* srcu_read_unlock() wakes! */ swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx])); WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */ + WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); /* Invoke the callbacks we removed above. */ while (lh) { -- 2.9.5
