Messages in this thread |  | | Subject | Re: [PATCH RFC tip/core/rcu 5/5] srcu: Provide polling interfaces for Tree SRCU grace periods | From | Neeraj Upadhyay <> | Date | Sun, 22 Nov 2020 19:52:58 +0530 |
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On 11/21/2020 5:46 AM, Paul E. McKenney wrote: > On Fri, Nov 20, 2020 at 05:31:43PM +0530, Neeraj Upadhyay wrote: >> >> >> On 11/17/2020 6:10 AM, paulmck@kernel.org wrote: >>> From: "Paul E. McKenney" <paulmck@kernel.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@gmail.com> >>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ] >>> Signed-off-by: Paul E. McKenney <paulmck@kernel.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? >
Looks good to me!
Thanks Neeraj
> 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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