On Fri, Nov 20, 2020 at 05:31:43PM +0530, Neeraj Upadhyay wrote: > > > On 11/17/2020 6:10 AM, paulmck@xxxxxxxxxx wrote: > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxx/ > > Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> > > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ] > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > --- > > 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; }
