From: Paul E. McKenney <paulmck@xxxxxxxxxx> commit 74612a07b83fc46c2b2e6f71a541d55b024ebefc upstream. 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.GF3365678@xxxxxxxxxxxxxx/ Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> [ paulmck: Fix ->srcu_lock_nesting[] indexing per Neeraj Upadhyay. ] Reviewed-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- include/linux/srcutiny.h | 6 +++--- kernel/rcu/srcutiny.c | 5 +++-- 2 files changed, 6 insertions(+), 5 deletions(-) --- 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(struc { int idx; - idx = READ_ONCE(ssp->srcu_idx); + idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1; WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1); return idx; } @@ -80,7 +80,7 @@ static inline void srcu_torture_stats_pr { int idx; - idx = READ_ONCE(ssp->srcu_idx) & 0x1; + idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1; pr_alert("%s%s Tiny SRCU per-CPU(idx=%d): (%hd,%hd)\n", tt, tf, idx, READ_ONCE(ssp->srcu_lock_nesting[!idx]), --- a/kernel/rcu/srcutiny.c +++ b/kernel/rcu/srcutiny.c @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *w 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) {
