On Wed, Nov 25, 2020 at 10:03:26AM +0530, Neeraj Upadhyay wrote: > > > On 11/24/2020 10:48 AM, Neeraj Upadhyay wrote: > > > > > > On 11/24/2020 1:25 AM, Paul E. McKenney wrote: > > > On Mon, Nov 23, 2020 at 10:01:13AM +0530, Neeraj Upadhyay wrote: > > > > On 11/21/2020 6:29 AM, paulmck@xxxxxxxxxx wrote: > > > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > > > > > > > > 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_read_lock() idx computation Neeraj per > > > > > Upadhyay. ] > > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > > --- > > > > > 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..d9edb67 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) + 1) & 0x2) >> 1; > > > > > WRITE_ONCE(ssp->srcu_lock_nesting[idx], > > > > > ssp->srcu_lock_nesting[idx] + 1); > > > > > return idx; > > > > > } > > > > > > > > Need change in idx calcultion in srcu_torture_stats_print() ? > > > > > > > > static inline void srcu_torture_stats_print(struct srcu_struct *ssp, > > > > idx = READ_ONCE(ssp->srcu_idx) & 0x1; > > > > > > Excellent point! It should match the calculation in __srcu_read_lock(), > > > shouldn't it? I have updated this, thank you! > > > > > > Thanx, Paul > > > > > > > Updated version looks good! > > > > > > Thanks > > Neeraj > > > > For the version in rcu -dev: > > Reviewed-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> I applied all of these, thank you very much! > Only minor point which I have is, the idx calculation can be made an inline > func (though srcu_drive_gp() does not require a READ_ONCE for ->srcu_idx): > > __srcu_read_lock() and srcu_torture_stats_print() are using > > idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1; > > whereas srcu_drive_gp() uses: > > idx = (ssp->srcu_idx & 0x2) / 2; They do work on different elements of the various arrays. Or do you believe that the srcu_drive_gp() use needs adjusting? Either way, the overhead of READ_ONCE() is absolutely not at all a problem. Would you like to put together a patch so that I can see exactly what you are suggesting? Thanx, Paul > Thanks > Neeraj > > > > > Thanks > > > > Neeraj > > > > > > > > > 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) { > > > > > > > > > > > > > -- > > > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is > > > > a member of > > > > the Code Aurora Forum, hosted by The Linux Foundation > > > > -- > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of > the Code Aurora Forum, hosted by The Linux Foundation
