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Date: Fri, 20 Nov 2020 17:31:43 +0530 From: Neeraj Upadhyay <neeraju@...eaurora.org> To: paulmck@...nel.org, rcu@...r.kernel.org Cc: 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 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? Thanks Neeraj > + return true; > +} > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu); > + > /* > * Callback function for srcu_barrier() use. > */ > -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation
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