srcu: Make Tiny SRCU use multi-bit grace-period counter

BugLink: https://bugs.launchpad.net/bugs/1944610
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@moria.home.lan/
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
[ paulmck: Fix ->srcu_lock_nesting[] indexing per Neeraj Upadhyay. ]
Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index 5a5a1941ca156d36270b37095bf7ef9f7c2247f1..b8b42d0a24f1179c197f2ef678d4687ce35d81c5 100644 (file)
--- 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) + 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_print(struct srcu_struct *ssp,
 {
        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]),

diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index ee25b365c3ead29a425307d7c43a28d8bbb757d3..3bac1db85a85b1018d0081721479b89ad648c670 100644 (file)
--- 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) {