From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9BA0944E665; Fri, 24 Jul 2026 18:31:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784917903; cv=none; b=eUPh3wes6urlR67A4rS162L2qEyFojLFChAEM9tdsqtg+WT/kG4gejOoDAga8KQPnm6Ne8WHC+IvSDM6+y+vaZZ/oStcDpdc5cp1hKRhx2sFKCBtwduFmQcmMNPG5YeN3GsL9wkbHgr2P4EPKdt2/G/Sbr2IEjHLzD3H6rJScDA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784917903; c=relaxed/simple; bh=w3HIgxpxdcezSe4tmw6xtnstaZxL8MLGuxujNFCudJU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=EFO3PTxnvTOJ1GWaBTWNcuEYieIJAfJDhas/pCW2StmTORz80BCYJc18cc+UsboM1psAcY14+uMX2M5kZNUg8TWRa6WtRhgX85NQTR1EWI3WYbgMyLpVa/5J4HnrBWSDA2mLFh+Q5QoGLz0XVUWS7D34Ktm5iATqcO5rQsVwBeM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Mm+GWFq7; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Mm+GWFq7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4B5BB1F000E9; Fri, 24 Jul 2026 18:31:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784917898; bh=Qy/LRN/B1QWzGPGEXcqfk0UJQ7PX/5WIAF5iAvdYdIU=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=Mm+GWFq7qVh7NG0j4btp1ZCeTxF8v4PTWrtApJfmq9wh/l5dMhVlArgEZkz0ql2iI ppneQDocpwvkPu5YuxhA27PMf1uRNFNEeunrLzO1jVaBBUbYDZOp8ByRqQhpJFaXYx WQVCjcm5jhlyyWsxzz6RD6jwmSITnXuY3ZNchEVYeiOOhElzks5WNO8ORJ7M6MT3Zb l8FAOlim0aalcHexW4SrAB2kDSNz7neXNZLjfhRgwKHEzOvlZlDwnyfCogJ25C2pnR 2W5oLq5ZbStO7r8sJlf6dhmnjK6FWM8EeGKcG1untuWpYYFt8XD4B1mgh9RxItC+/q 34aGm2FcZPLFQ== From: Puranjay Mohan To: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Cc: "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Masami Hiramatsu , Davidlohr Bueso , Breno Leitao , Puranjay Mohan Subject: [PATCH v2 6/11] rcu: Enable RCU callbacks to benefit from expedited grace periods Date: Fri, 24 Jul 2026 11:31:23 -0700 Message-ID: <20260724183123.160051-1-puranjay@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260624132356.516959-7-puranjay@kernel.org> References: <20260624132356.516959-7-puranjay@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Currently, RCU callbacks only track normal grace-period sequence numbers. This means callbacks must wait for normal grace periods to complete even when expedited grace periods have already elapsed. Use the full struct rcu_gp_seq (which tracks both the normal and expedited grace-period sequences) throughout the callback infrastructure. rcu_segcblist_advance() now checks both normal and expedited GP completion via poll_state_synchronize_rcu_full(), and becomes parameterless since it reads the grace-period state internally. rcu_segcblist_accelerate() stores the full state (both sequences) instead of just the normal one. rcu_accelerate_cbs() and rcu_accelerate_cbs_unlocked() use get_state_synchronize_rcu_full() to capture both sequences, and the NOCB advance checks use poll_state_synchronize_rcu_full() instead of comparing only the normal sequence. srcu_segcblist_advance() becomes a standalone implementation because it compares SRCU sequences directly and cannot use poll_state_synchronize_rcu_full(), which reads RCU-specific globals. srcu_segcblist_accelerate() sets the ->exp field to RCU_GET_STATE_NOT_TRACKED so that poll_state_synchronize_rcu_full() compares only ->norm and ignores ->exp. Signed-off-by: Puranjay Mohan --- Paul, please take this in place of the original 6/11. The only change from v1 is dropping the premature !poll_state_synchronize_rcu_full() gate in nocb_gp_wait(): if a grace period completed between rcu_advance_cbs() and that poll, the rcuog slept grace-period-deaf in nocb_gp_sleep() with advanceable callbacks still queued -- the lost wakeup Frederic diagnosed. Verified by widening that window with a delay: the original reproduced the writer stall within ~75s, this version stayed clean. kernel/rcu/rcu_segcblist.c | 30 +++++++++++++++++++++++------- kernel/rcu/rcu_segcblist.h | 2 +- kernel/rcu/tree.c | 9 +++------ kernel/rcu/tree_nocb.h | 22 ++++++++++++---------- 4 files changed, 39 insertions(+), 24 deletions(-) diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c index 4e3dfe42bc097..cf8951d33e767 100644 --- a/kernel/rcu/rcu_segcblist.c +++ b/kernel/rcu/rcu_segcblist.c @@ -12,6 +12,7 @@ #include #include +#include "rcu.h" #include "rcu_segcblist.h" /* Initialize simple callback list. */ @@ -494,9 +495,9 @@ static void rcu_segcblist_advance_compact(struct rcu_segcblist *rsclp, int i) /* * Advance the callbacks in the specified rcu_segcblist structure based - * on the current value passed in for the grace-period counter. + * on the current value of the grace-period counter. */ -void rcu_segcblist_advance(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gsp) +void rcu_segcblist_advance(struct rcu_segcblist *rsclp) { int i; @@ -509,7 +510,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gsp) * are ready to invoke, and put them into the RCU_DONE_TAIL segment. */ for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) { - if (ULONG_CMP_LT(gsp->norm, rsclp->gp_seq[i].norm)) + if (!poll_state_synchronize_rcu_full(&rsclp->gp_seq[i])) break; WRITE_ONCE(rsclp->tails[RCU_DONE_TAIL], rsclp->tails[i]); rcu_segcblist_move_seglen(rsclp, i, RCU_DONE_TAIL); @@ -595,7 +596,7 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gs */ for (; i < RCU_NEXT_TAIL; i++) { WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_NEXT_TAIL]); - rsclp->gp_seq[i].norm = gsp->norm; + rsclp->gp_seq[i] = *gsp; } return true; } @@ -637,14 +638,29 @@ void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp, void srcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq) { - struct rcu_gp_seq gs = { .norm = seq }; + int i; + + WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp)); + if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL)) + return; + + for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) { + if (ULONG_CMP_LT(seq, rsclp->gp_seq[i].norm)) + break; + WRITE_ONCE(rsclp->tails[RCU_DONE_TAIL], rsclp->tails[i]); + rcu_segcblist_move_seglen(rsclp, i, RCU_DONE_TAIL); + } + + /* If no callbacks moved, nothing more need be done. */ + if (i == RCU_WAIT_TAIL) + return; - rcu_segcblist_advance(rsclp, &gs); + rcu_segcblist_advance_compact(rsclp, i); } bool srcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq) { - struct rcu_gp_seq gs = { .norm = seq }; + struct rcu_gp_seq gs = { .norm = seq, .exp = RCU_GET_STATE_NOT_TRACKED }; return rcu_segcblist_accelerate(rsclp, &gs); } diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h index 16b0cb6b32507..431c4466b8898 100644 --- a/kernel/rcu/rcu_segcblist.h +++ b/kernel/rcu/rcu_segcblist.h @@ -139,7 +139,7 @@ void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp, struct rcu_cblist *rclp); void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp, struct rcu_cblist *rclp); -void rcu_segcblist_advance(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gsp); +void rcu_segcblist_advance(struct rcu_segcblist *rsclp); bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, struct rcu_gp_seq *gsp); void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp, struct rcu_segcblist *src_rsclp); diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 640ebe055a246..f63ce75c5b4f5 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1159,7 +1159,7 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp) * accelerating callback invocation to an earlier grace-period * number. */ - gs.norm = rcu_seq_snap(&rcu_state.gp_seq); + get_state_synchronize_rcu_full(&gs); if (rcu_segcblist_accelerate(&rdp->cblist, &gs)) ret = rcu_start_this_gp(rnp, rdp, gs.norm); @@ -1188,7 +1188,7 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp, bool needwake; rcu_lockdep_assert_cblist_protected(rdp); - gs.norm = rcu_seq_snap(&rcu_state.gp_seq); + get_state_synchronize_rcu_full(&gs); if (!READ_ONCE(rdp->gpwrap) && ULONG_CMP_GE(rdp->gp_seq_needed, gs.norm)) { /* Old request still live, so mark recent callbacks. */ (void)rcu_segcblist_accelerate(&rdp->cblist, &gs); @@ -1213,8 +1213,6 @@ static void rcu_accelerate_cbs_unlocked(struct rcu_node *rnp, */ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp) { - struct rcu_gp_seq gs; - rcu_lockdep_assert_cblist_protected(rdp); raw_lockdep_assert_held_rcu_node(rnp); @@ -1226,8 +1224,7 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp) * Find all callbacks whose ->gp_seq numbers indicate that they * are ready to invoke, and put them into the RCU_DONE_TAIL sublist. */ - gs.norm = rnp->gp_seq; - rcu_segcblist_advance(&rdp->cblist, &gs); + rcu_segcblist_advance(&rdp->cblist); /* Classify any remaining callbacks. */ return rcu_accelerate_cbs(rnp, rdp); diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index eaa1ad5297e29..876dd897f18be 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -502,7 +502,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, } if (j != rdp->nocb_gp_adv_time && rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) && - rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq.norm)) { + poll_state_synchronize_rcu_full(&cur_gp_seq)) { rcu_advance_cbs_nowake(rdp->mynode, rdp); rdp->nocb_gp_adv_time = j; } @@ -731,17 +731,17 @@ static noinline_for_stack void nocb_gp_wait(struct rcu_data *my_rdp) if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL) || (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) && - rcu_seq_done(&rnp->gp_seq, cur_gp_seq.norm))) { + poll_state_synchronize_rcu_full(&cur_gp_seq))) { raw_spin_lock_rcu_node(rnp); /* irqs disabled. */ needwake_gp = rcu_advance_cbs(rnp, rdp); wasempty = rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL); raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */ } - // Need to wait on some grace period? WARN_ON_ONCE(wasempty && !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL)); + // Need to wait on some grace period? if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) { if (!needwait_gp || ULONG_CMP_LT(cur_gp_seq.norm, wait_gp_seq)) @@ -919,7 +919,7 @@ static void nocb_cb_wait(struct rcu_data *rdp) lockdep_assert_irqs_enabled(); rcu_nocb_lock_irqsave(rdp, flags); if (rcu_segcblist_nextgp(cblist, &cur_gp_seq) && - rcu_seq_done(&rnp->gp_seq, cur_gp_seq.norm) && + poll_state_synchronize_rcu_full(&cur_gp_seq) && raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */ needwake_gp = rcu_advance_cbs(rdp->mynode, rdp); raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */ @@ -1548,8 +1548,8 @@ static void show_rcu_nocb_gp_state(struct rcu_data *rdp) static void show_rcu_nocb_state(struct rcu_data *rdp) { char bufd[22]; - char bufw[45]; - char bufr[45]; + char bufw[64]; + char bufr[64]; char bufn[22]; char bufb[22]; struct rcu_data *nocb_next_rdp; @@ -1569,10 +1569,12 @@ static void show_rcu_nocb_state(struct rcu_data *rdp) nocb_entry_rdp); sprintf(bufd, "%ld", rsclp->seglen[RCU_DONE_TAIL]); - sprintf(bufw, "%ld(%ld)", rsclp->seglen[RCU_WAIT_TAIL], - rsclp->gp_seq[RCU_WAIT_TAIL].norm); - sprintf(bufr, "%ld(%ld)", rsclp->seglen[RCU_NEXT_READY_TAIL], - rsclp->gp_seq[RCU_NEXT_READY_TAIL].norm); + sprintf(bufw, "%ld(%ld/%ld)", rsclp->seglen[RCU_WAIT_TAIL], + rsclp->gp_seq[RCU_WAIT_TAIL].norm, + rsclp->gp_seq[RCU_WAIT_TAIL].exp); + sprintf(bufr, "%ld(%ld/%ld)", rsclp->seglen[RCU_NEXT_READY_TAIL], + rsclp->gp_seq[RCU_NEXT_READY_TAIL].norm, + rsclp->gp_seq[RCU_NEXT_READY_TAIL].exp); sprintf(bufn, "%ld", rsclp->seglen[RCU_NEXT_TAIL]); sprintf(bufb, "%ld", rcu_cblist_n_cbs(&rdp->nocb_bypass)); pr_info(" CB %d^%d->%d %c%c%c%c%c F%ld L%ld C%d %c%s%c%s%c%s%c%s%c%s q%ld %c CPU %d%s\n", -- 2.53.0-Meta