Re: [PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter
On Thu, Nov 19, 2020 at 01:44:49PM +0530, Neeraj Upadhyay wrote:
> Hi Paul,
>
> On 11/17/2020 6:10 AM, paul...@kernel.org wrote:
> > From: "Paul E. McKenney"
> >
> > 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
> > Signed-off-by: Paul E. McKenney
> > ---
> > 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;
>
> Should we use bit 0x2 of (READ_ONCE(ssp->srcu_idx) + 1) , if GP
> (srcu_drive_gp()) is in progress? Or am I missing something here?
>
> idx = ((READ_ONCE(ssp->srcu_idx) +1) & 0x2) / 2;
You miss nothing! I am running about 200 hours of concurrent rcutorture
of the SRCU-t and SRCU-u scenarios, but I must admit that this race could
be hard to hit. But it could of course result in too-short grace periods.
I will fold this into the original with attribution.
> Also, any reason for using divison instead of shift; something to
> do with 16-bit srcu_idx which I am missing here?
I just figure that the compiler is better at selecting instructions
than I am. Either would work.
Thanx, Paul
> Thanks
> Neeraj
>
> > 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 = >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) {
> >
>
> --
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation
Re: [PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter
Hi Paul,
On 11/17/2020 6:10 AM, paul...@kernel.org wrote:
From: "Paul E. McKenney"
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
Signed-off-by: Paul E. McKenney
---
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;
Should we use bit 0x2 of (READ_ONCE(ssp->srcu_idx) + 1) , if GP
(srcu_drive_gp()) is in progress? Or am I missing something here?
idx = ((READ_ONCE(ssp->srcu_idx) +1) & 0x2) / 2;
Also, any reason for using divison instead of shift; something to
do with 16-bit srcu_idx which I am missing here?
Thanks
Neeraj
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 = >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) {
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation
[PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter
From: "Paul E. McKenney"
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
Signed-off-by: Paul E. McKenney
---
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 = >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
