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SubjectRe: [PATCH 3/3 RFC paul/rcu/srcu] srcu: flip only once for every grace period
On Fri, Feb 24, 2012 at 04:06:01PM +0800, Lai Jiangshan wrote:
> On 02/22/2012 05:29 PM, Lai Jiangshan wrote:
> >>From 4ddf62aaf2c4ebe6b9d4a1c596e8b43a678f1f0d Mon Sep 17 00:00:00 2001
> > From: Lai Jiangshan <laijs@cn.fujitsu.com>
> > Date: Wed, 22 Feb 2012 14:12:02 +0800
> > Subject: [PATCH 3/3 RFC paul/rcu/srcu] srcu: flip only once for every grace period
> >
> > flip_idx_and_wait() is not changed, and is split as two functions
> > and only a short comments is added for smp_mb() E.
> >
> > __synchronize_srcu() use a different algorithm for "leak" readers.
> > detail is in the comments of the patch.
> >
> > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > ---
> > kernel/srcu.c | 105 ++++++++++++++++++++++++++++++++++----------------------
> > 1 files changed, 64 insertions(+), 41 deletions(-)
> >
> > diff --git a/kernel/srcu.c b/kernel/srcu.c
> > index a51ac48..346f9d7 100644
> > --- a/kernel/srcu.c
> > +++ b/kernel/srcu.c
> > @@ -249,6 +249,37 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> > */
> > #define SYNCHRONIZE_SRCU_READER_DELAY 5
> >
> > +static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
> > +{
> > + int trycount = 0;
>
> Hi, Paul
>
> smp_mb() D also needs to be moved here, could you fix it before push it.
> I thought it(smp_mb()) always here in my mind, wrong assumption.

Good catch -- I should have seen this myself. I committed this and pushed
it to:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/srcu

> Sorry.

Not a problem, though it does point out the need for review and testing.
So I am feeling a bit nervous about pushing this into 3.4, and am
beginning to think that it needs mechanical proof as well as more testing.

Thoughts?

Thanx, Paul

> Thanks,
> Lai
>
> > +
> > + /*
> > + * SRCU read-side critical sections are normally short, so wait
> > + * a small amount of time before possibly blocking.
> > + */
> > + if (!srcu_readers_active_idx_check(sp, idx)) {
> > + udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> > + while (!srcu_readers_active_idx_check(sp, idx)) {
> > + if (expedited && ++ trycount < 10)
> > + udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> > + else
> > + schedule_timeout_interruptible(1);
> > + }
> > + }
> > +
> > + /*
> > + * The following smp_mb() E pairs with srcu_read_unlock()'s
> > + * smp_mb C to ensure that if srcu_readers_active_idx_check()
> > + * sees srcu_read_unlock()'s counter decrement, then any
> > + * of the current task's subsequent code will happen after
> > + * that SRCU read-side critical section.
> > + *
> > + * It also ensures the order between the above waiting and
> > + * the next flipping.
> > + */
> > + smp_mb(); /* E */
> > +}
> > +
> > /*
> > * Flip the readers' index by incrementing ->completed, then wait
> > * until there are no more readers using the counters referenced by
> > @@ -258,12 +289,12 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> > * Of course, it is possible that a reader might be delayed for the
> > * full duration of flip_idx_and_wait() between fetching the
> > * index and incrementing its counter. This possibility is handled
> > - * by __synchronize_srcu() invoking flip_idx_and_wait() twice.
> > + * by the next __synchronize_srcu() invoking wait_idx() for such readers
> > + * before start a new grace perioad.
> > */
> > static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
> > {
> > int idx;
> > - int trycount = 0;
> >
> > idx = sp->completed++ & 0x1;
> >
> > @@ -278,28 +309,7 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
> > */
> > smp_mb(); /* D */
> >
> > - /*
> > - * SRCU read-side critical sections are normally short, so wait
> > - * a small amount of time before possibly blocking.
> > - */
> > - if (!srcu_readers_active_idx_check(sp, idx)) {
> > - udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> > - while (!srcu_readers_active_idx_check(sp, idx)) {
> > - if (expedited && ++ trycount < 10)
> > - udelay(SYNCHRONIZE_SRCU_READER_DELAY);
> > - else
> > - schedule_timeout_interruptible(1);
> > - }
> > - }
> > -
> > - /*
> > - * The following smp_mb() E pairs with srcu_read_unlock()'s
> > - * smp_mb C to ensure that if srcu_readers_active_idx_check()
> > - * sees srcu_read_unlock()'s counter decrement, then any
> > - * of the current task's subsequent code will happen after
> > - * that SRCU read-side critical section.
> > - */
> > - smp_mb(); /* E */
> > + wait_idx(sp, idx, expedited);
> > }
> >
> > /*
> > @@ -307,8 +317,6 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
> > */
> > static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
> > {
> > - int idx = 0;
> > -
> > rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
> > !lock_is_held(&rcu_bh_lock_map) &&
> > !lock_is_held(&rcu_lock_map) &&
> > @@ -318,27 +326,42 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
> > mutex_lock(&sp->mutex);
> >
> > /*
> > - * If there were no helpers, then we need to do two flips of
> > - * the index. The first flip is required if there are any
> > - * outstanding SRCU readers even if there are no new readers
> > - * running concurrently with the first counter flip.
> > - *
> > - * The second flip is required when a new reader picks up
> > + * When in the previous grace perioad, if a reader picks up
> > * the old value of the index, but does not increment its
> > * counter until after its counters is summed/rechecked by
> > - * srcu_readers_active_idx_check(). In this case, the current SRCU
> > + * srcu_readers_active_idx_check(). In this case, the previous SRCU
> > * grace period would be OK because the SRCU read-side critical
> > - * section started after this SRCU grace period started, so the
> > + * section started after the SRCU grace period started, so the
> > * grace period is not required to wait for the reader.
> > *
> > - * However, the next SRCU grace period would be waiting for the
> > - * other set of counters to go to zero, and therefore would not
> > - * wait for the reader, which would be very bad. To avoid this
> > - * bad scenario, we flip and wait twice, clearing out both sets
> > - * of counters.
> > + * However, such leftover readers affect this new SRCU grace period.
> > + * So we have to wait for such readers. This wait_idx() should be
> > + * considerred as the wait_idx() in the flip_idx_and_wait() of
> > + * the previous grace perioad except that it is for leftover readers
> > + * started before this synchronize_srcu(). So when it returns,
> > + * there is no leftover readers that starts before this grace period.
> > + *
> > + * If there are some leftover readers that do not increment its
> > + * counter until after its counters is summed/rechecked by
> > + * srcu_readers_active_idx_check(), In this case, this SRCU
> > + * grace period would be OK as above comments says. We defines
> > + * such readers as leftover-leftover readers, we consider these
> > + * readers fteched index of (sp->completed + 1), it means they
> > + * are considerred as exactly the same as the readers after this
> > + * grace period.
> > + *
> > + * wait_idx() is expected very fast, because leftover readers
> > + * are unlikely produced.
> > */
> > - for (; idx < 2; idx++)
> > - flip_idx_and_wait(sp, expedited);
> > + wait_idx(sp, (sp->completed - 1) & 0x1, expedited);
> > +
> > + /*
> > + * Starts a new grace period, this flip is required if there are
> > + * any outstanding SRCU readers even if there are no new readers
> > + * running concurrently with the counter flip.
> > + */
> > + flip_idx_and_wait(sp, expedited);
> > +
> > mutex_unlock(&sp->mutex);
> > }
> >
>



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