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Message-ID: <20201121001602.GO1437@paulmck-ThinkPad-P72>
Date: Fri, 20 Nov 2020 16:16:02 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 5/5] srcu: Provide polling interfaces
for Tree SRCU grace periods
On Fri, Nov 20, 2020 at 05:31:43PM +0530, Neeraj Upadhyay wrote:
>
>
> On 11/17/2020 6:10 AM, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> >
> > There is a need for a polling interface for SRCU grace
> > periods, so this commit supplies get_state_synchronize_srcu(),
> > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> > purpose. The first can be used if future grace periods are inevitable
> > (perhaps due to a later call_srcu() invocation), the second if future
> > grace periods might not otherwise happen, and the third to check if a
> > grace period has elapsed since the corresponding call to either of the
> > first two.
> >
> > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> > the return value from either get_state_synchronize_srcu() or
> > start_poll_synchronize_srcu() must be passed in to a later call to
> > poll_state_synchronize_srcu().
> >
> > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> > Reported-by: Kent Overstreet <kent.overstreet@...il.com>
> > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> > kernel/rcu/srcutree.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++---
> > 1 file changed, 60 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index d930ece..015d80e 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -810,7 +810,8 @@ static void srcu_leak_callback(struct rcu_head *rhp)
> > /*
> > * Start an SRCU grace period, and also queue the callback if non-NULL.
> > */
> > -static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rhp, bool do_norm)
> > +static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > + struct rcu_head *rhp, bool do_norm)
> > {
> > unsigned long flags;
> > int idx;
> > @@ -822,7 +823,8 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
> > idx = srcu_read_lock(ssp);
> > sdp = raw_cpu_ptr(ssp->sda);
> > spin_lock_irqsave_rcu_node(sdp, flags);
> > - rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> > + if (rhp)
> > + rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> > rcu_segcblist_advance(&sdp->srcu_cblist,
> > rcu_seq_current(&ssp->srcu_gp_seq));
> > s = rcu_seq_snap(&ssp->srcu_gp_seq);
> > @@ -841,6 +843,7 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
> > else if (needexp)
> > srcu_funnel_exp_start(ssp, sdp->mynode, s);
> > srcu_read_unlock(ssp, idx);
> > + return s;
> > }
> > /*
> > @@ -882,7 +885,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> > return;
> > }
> > rhp->func = func;
> > - srcu_gp_start_if_needed(ssp, rhp, do_norm);
> > + (void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
> > }
> > /**
> > @@ -1011,6 +1014,60 @@ void synchronize_srcu(struct srcu_struct *ssp)
> > }
> > EXPORT_SYMBOL_GPL(synchronize_srcu);
> > +/**
> > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> > + * @ssp: srcu_struct to provide cookie for.
> > + *
> > + * This function returns a cookie that can be passed to
> > + * poll_state_synchronize_srcu(), which will return true if a full grace
> > + * period has elapsed in the meantime. It is the caller's responsibility
> > + * to make sure that grace period happens, for example, by invoking
> > + * call_srcu() after return from get_state_synchronize_srcu().
> > + */
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > + // Any prior manipulation of SRCU-protected data must happen
> > + // before the load from ->srcu_gp_seq.
> > + smp_mb();
> > + return rcu_seq_snap(&ssp->srcu_gp_seq);
> > +}
> > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> > +
> > +/**
> > + * start_poll_synchronize_srcu - Provide cookie and start grace period
> > + * @ssp: srcu_struct to provide cookie for.
> > + *
> > + * This function returns a cookie that can be passed to
> > + * poll_state_synchronize_srcu(), which will return true if a full grace
> > + * period has elapsed in the meantime. Unlike get_state_synchronize_srcu(),
> > + * this function also ensures that any needed SRCU grace period will be
> > + * started. This convenience does come at a cost in terms of CPU overhead.
> > + */
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > + return srcu_gp_start_if_needed(ssp, NULL, true);
> > +}
> > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> > +
> > +/**
> > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> > + * @ssp: srcu_struct to provide cookie for.
> > + * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
> > + *
> > + * This function takes the cookie that was returned from either
> > + * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
> > + * returns @true if an SRCU grace period elapsed since the time that the
> > + * cookie was created.
> > + */
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> > +{
> > + if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
> > + return false;
> > + smp_mb(); // ^^^
>
> Minor: Should this comment be more descriptive?
You have to read between the lines? ;-)
How about like this?
Thanx, Paul
------------------------------------------------------------------------
bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
{
if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
return false;
// Ensure that the end of the SRCU grace period happens before
// any subsequent code that the caller might execute.
smp_mb(); // ^^^
return true;
}
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