From: Frederic Weisbecker frederic@kernel.org

mainline inclusion from mainline-v6.8-rc2 commit e787644caf7628ad3269c1fbd321c3255cf51710 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I9NZ3E

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id...

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When the CPU goes idle for the last time during the CPU down hotplug process, RCU reports a final quiescent state for the current CPU. If this quiescent state propagates up to the top, some tasks may then be woken up to complete the grace period: the main grace period kthread and/or the expedited main workqueue (or kworker).

If those kthreads have a SCHED_FIFO policy, the wake up can indirectly arm the RT bandwith timer to the local offline CPU. Since this happens after hrtimers have been migrated at CPUHP_AP_HRTIMERS_DYING stage, the timer gets ignored. Therefore if the RCU kthreads are waiting for RT bandwidth to be available, they may never be actually scheduled.

This triggers TREE03 rcutorture hangs:

rcu: INFO: rcu_preempt self-detected stall on CPU rcu: 4-...!: (1 GPs behind) idle=9874/1/0x4000000000000000 softirq=0/0 fqs=20 rcuc=21071 jiffies(starved) rcu: (t=21035 jiffies g=938281 q=40787 ncpus=6) rcu: rcu_preempt kthread starved for 20964 jiffies! g938281 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0 rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior. rcu: RCU grace-period kthread stack dump: task:rcu_preempt state:R running task stack:14896 pid:14 tgid:14 ppid:2 flags:0x00004000 Call Trace: <TASK> __schedule+0x2eb/0xa80 schedule+0x1f/0x90 schedule_timeout+0x163/0x270 ? __pfx_process_timeout+0x10/0x10 rcu_gp_fqs_loop+0x37c/0x5b0 ? __pfx_rcu_gp_kthread+0x10/0x10 rcu_gp_kthread+0x17c/0x200 kthread+0xde/0x110 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x2b/0x40 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1b/0x30 </TASK>

The situation can't be solved with just unpinning the timer. The hrtimer infrastructure and the nohz heuristics involved in finding the best remote target for an unpinned timer would then also need to handle enqueues from an offline CPU in the most horrendous way.

So fix this on the RCU side instead and defer the wake up to an online CPU if it's too late for the local one.

Reported-by: Paul E. McKenney paulmck@kernel.org Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Neeraj Upadhyay (AMD) neeraj.iitr10@gmail.com Signed-off-by: Wei Li liwei391@huawei.com --- kernel/rcu/tree.c | 34 +++++++++++++++++++++++++++++++++- kernel/rcu/tree_exp.h | 3 +-- 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 81e3550ea6f3..d92e55e3478d 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1483,6 +1483,38 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp) return needmore; }

+static void swake_up_one_online_ipi(void *arg) +{ + struct swait_queue_head *wqh = arg; + + swake_up_one(wqh); +} + +static void swake_up_one_online(struct swait_queue_head *wqh) +{ + int cpu = get_cpu(); + + /* + * If called from rcutree_report_cpu_starting(), wake up + * is dangerous that late in the CPU-down hotplug process. The + * scheduler might queue an ignored hrtimer. Defer the wake up + * to an online CPU instead. + */ + if (unlikely(cpu_is_offline(cpu))) { + int target; + + target = cpumask_any_and(housekeeping_cpumask(HK_FLAG_RCU), + cpu_online_mask); + + smp_call_function_single(target, swake_up_one_online_ipi, + wqh, 0); + put_cpu(); + } else { + put_cpu(); + swake_up_one(wqh); + } +} + /* * Awaken the grace-period kthread. Don't do a self-awaken (unless in an * interrupt or softirq handler, in which case we just might immediately @@ -1507,7 +1539,7 @@ static void rcu_gp_kthread_wake(void) return; WRITE_ONCE(rcu_state.gp_wake_time, jiffies); WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq)); - swake_up_one(&rcu_state.gp_wq); + swake_up_one_online(&rcu_state.gp_wq); }

/* diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 07a284a18645..8394f193f06f 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -170,7 +170,6 @@ static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp) return ret; }

- /* * Report the exit from RCU read-side critical section for the last task * that queued itself during or before the current expedited preemptible-RCU @@ -198,7 +197,7 @@ static void __rcu_report_exp_rnp(struct rcu_node *rnp, raw_spin_unlock_irqrestore_rcu_node(rnp, flags); if (wake) { smp_mb(); /* EGP done before wake_up(). */ - swake_up_one(&rcu_state.expedited_wq); + swake_up_one_online(&rcu_state.expedited_wq); } break; }

From: Frederic Weisbecker frederic@kernel.org

mainline inclusion from mainline-v6.7-rc1 commit 4a8e65b0c348e42107c64381e692e282900be361 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I9NZ3E

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id...

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SRCU callbacks acceleration might fail if the preceding callbacks advance also fails. This can happen when the following steps are met:

1) The RCU_WAIT_TAIL segment has callbacks (say for gp_num 8) and the RCU_NEXT_READY_TAIL also has callbacks (say for gp_num 12).

2) The grace period for RCU_WAIT_TAIL is observed as started but not yet completed so rcu_seq_current() returns 4 + SRCU_STATE_SCAN1 = 5.

3) This value is passed to rcu_segcblist_advance() which can't move any segment forward and fails.

4) srcu_gp_start_if_needed() still proceeds with callback acceleration. But then the call to rcu_seq_snap() observes the grace period for the RCU_WAIT_TAIL segment (gp_num 8) as completed and the subsequent one for the RCU_NEXT_READY_TAIL segment as started (ie: 8 + SRCU_STATE_SCAN1 = 9) so it returns a snapshot of the next grace period, which is 16.

5) The value of 16 is passed to rcu_segcblist_accelerate() but the freshly enqueued callback in RCU_NEXT_TAIL can't move to RCU_NEXT_READY_TAIL which already has callbacks for a previous grace period (gp_num = 12). So acceleration fails.

6) Note in all these steps, srcu_invoke_callbacks() hadn't had a chance to run srcu_invoke_callbacks().

Then some very bad outcome may happen if the following happens:

7) Some other CPU races and starts the grace period number 16 before the CPU handling previous steps had a chance. Therefore srcu_gp_start() isn't called on the latter sdp to fix the acceleration leak from previous steps with a new pair of call to advance/accelerate.

8) The grace period 16 completes and srcu_invoke_callbacks() is finally called. All the callbacks from previous grace periods (8 and 12) are correctly advanced and executed but callbacks in RCU_NEXT_READY_TAIL still remain. Then rcu_segcblist_accelerate() is called with a snaphot of 20.

9) Since nothing started the grace period number 20, callbacks stay unhandled.

This has been reported in real load:

[3144162.608392] INFO: task kworker/136:12:252684 blocked for more than 122 seconds. [3144162.615986] Tainted: G O K 5.4.203-1-tlinux4-0011.1 #1 [3144162.623053] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [3144162.631162] kworker/136:12 D 0 252684 2 0x90004000 [3144162.631189] Workqueue: kvm-irqfd-cleanup irqfd_shutdown [kvm] [3144162.631192] Call Trace: [3144162.631202] __schedule+0x2ee/0x660 [3144162.631206] schedule+0x33/0xa0 [3144162.631209] schedule_timeout+0x1c4/0x340 [3144162.631214] ? update_load_avg+0x82/0x660 [3144162.631217] ? raw_spin_rq_lock_nested+0x1f/0x30 [3144162.631218] wait_for_completion+0x119/0x180 [3144162.631220] ? wake_up_q+0x80/0x80 [3144162.631224] __synchronize_srcu.part.19+0x81/0xb0 [3144162.631226] ? __bpf_trace_rcu_utilization+0x10/0x10 [3144162.631227] synchronize_srcu+0x5f/0xc0 [3144162.631236] irqfd_shutdown+0x3c/0xb0 [kvm] [3144162.631239] ? __schedule+0x2f6/0x660 [3144162.631243] process_one_work+0x19a/0x3a0 [3144162.631244] worker_thread+0x37/0x3a0 [3144162.631247] kthread+0x117/0x140 [3144162.631247] ? process_one_work+0x3a0/0x3a0 [3144162.631248] ? __kthread_cancel_work+0x40/0x40 [3144162.631250] ret_from_fork+0x1f/0x30

Fix this with taking the snapshot for acceleration _before_ the read of the current grace period number.

The only side effect of this solution is that callbacks advancing happen then _after_ the full barrier in rcu_seq_snap(). This is not a problem because that barrier only cares about:

1) Ordering accesses of the update side before call_srcu() so they don't bleed. 2) See all the accesses prior to the grace period of the current gp_num

The only things callbacks advancing need to be ordered against are carried by snp locking.

Reported-by: Yong He alexyonghe@tencent.com Co-developed-by:: Yong He alexyonghe@tencent.com Signed-off-by: Yong He alexyonghe@tencent.com Co-developed-by: Joel Fernandes (Google) joel@joelfernandes.org Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org Co-developed-by: Neeraj upadhyay Neeraj.Upadhyay@amd.com Signed-off-by: Neeraj upadhyay Neeraj.Upadhyay@amd.com Link: http://lore.kernel.org/CANZk6aR+CqZaqmMWrC2eRRPY12qAZnDZLwLnHZbNi=xXMB401g@m... Fixes: da915ad5cf25 ("srcu: Parallelize callback handling") Signed-off-by: Frederic Weisbecker frederic@kernel.org Conflicts: kernel/rcu/srcutree.c [adapt ssp->srcu_sup->srcu_gp_seq to ssp->srcu_gp_seq] Signed-off-by: Wei Li liwei391@huawei.com --- kernel/rcu/srcutree.c | 31 +++++++++++++++++++++++++++++-- 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 1ca734767b9d..464636475636 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -829,10 +829,37 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, spin_lock_irqsave_rcu_node(sdp, flags); if (rhp) rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp); + /* + * The snapshot for acceleration must be taken _before_ the read of the + * current gp sequence used for advancing, otherwise advancing may fail + * and acceleration may then fail too. + * + * This could happen if: + * + * 1) The RCU_WAIT_TAIL segment has callbacks (gp_num = X + 4) and the + * RCU_NEXT_READY_TAIL also has callbacks (gp_num = X + 8). + * + * 2) The grace period for RCU_WAIT_TAIL is seen as started but not + * completed so rcu_seq_current() returns X + SRCU_STATE_SCAN1. + * + * 3) This value is passed to rcu_segcblist_advance() which can't move + * any segment forward and fails. + * + * 4) srcu_gp_start_if_needed() still proceeds with callback acceleration. + * But then the call to rcu_seq_snap() observes the grace period for the + * RCU_WAIT_TAIL segment as completed and the subsequent one for the + * RCU_NEXT_READY_TAIL segment as started (ie: X + 4 + SRCU_STATE_SCAN1) + * so it returns a snapshot of the next grace period, which is X + 12. + * + * 5) The value of X + 12 is passed to rcu_segcblist_accelerate() but the + * freshly enqueued callback in RCU_NEXT_TAIL can't move to + * RCU_NEXT_READY_TAIL which already has callbacks for a previous grace + * period (gp_num = X + 8). So acceleration fails. + */ + s = rcu_seq_snap(&ssp->srcu_gp_seq); rcu_segcblist_advance(&sdp->srcu_cblist, rcu_seq_current(&ssp->srcu_gp_seq)); - s = rcu_seq_snap(&ssp->srcu_gp_seq); - (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s); + WARN_ON_ONCE(!rcu_segcblist_accelerate(&sdp->srcu_cblist, s) && rhp); if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) { sdp->srcu_gp_seq_needed = s; needgp = true;