From: Joel Fernandes <joel@joelfernandes.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
rushikesh.s.kadam@intel.com, urezki@gmail.com,
neeraj.iitr10@gmail.com, paulmck@kernel.org, rostedt@goodmis.org,
vineeth@bitbyteword.org, boqun.feng@gmail.com
Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation
Date: Sat, 3 Sep 2022 22:00:29 +0000 [thread overview]
Message-ID: <YxPOfVMzRWEa7xqf@google.com> (raw)
In-Reply-To: <20220902152132.GA115525@lothringen>
On Fri, Sep 02, 2022 at 05:21:32PM +0200, Frederic Weisbecker wrote:
[..]
> > +
> > raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
> >
> > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
> [...]
> > @@ -705,12 +816,21 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> > my_rdp->nocb_gp_gp = needwait_gp;
> > my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
> >
> > - if (bypass && !rcu_nocb_poll) {
> > - // At least one child with non-empty ->nocb_bypass, so set
> > - // timer in order to avoid stranding its callbacks.
> > - wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_BYPASS,
> > - TPS("WakeBypassIsDeferred"));
> > + // At least one child with non-empty ->nocb_bypass, so set
> > + // timer in order to avoid stranding its callbacks.
> > + if (!rcu_nocb_poll) {
> > + // If bypass list only has lazy CBs. Add a deferred
> > + // lazy wake up.
> > + if (lazy && !bypass) {
> > + wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_LAZY,
> > + TPS("WakeLazyIsDeferred"));
>
> What if:
>
> 1) rdp(1) has lazy callbacks
> 2) all other rdp's have no callback at all
> 3) nocb_gp_wait() runs through all rdp's, everything is handled, except for
> these lazy callbacks
> 4) It reaches the above path, ready to arm the RCU_NOCB_WAKE_LAZY timer,
> but it hasn't yet called wake_nocb_gp_defer()
> 5) Oh but rdp(2) queues a non-lazy callback. interrupts are disabled so it defers
> the wake up to nocb_gp_wait() with arming the timer in RCU_NOCB_WAKE.
> 6) nocb_gp_wait() finally calls wake_nocb_gp_defer() and override the timeout
> to several seconds ahead.
> 7) No more callbacks queued, the non-lazy callback will have to wait several
> seconds to complete.
>
> Or did I miss something? Note that the race exists with RCU_NOCB_WAKE_BYPASS
> but it's only about one jiffy delay, not seconds.
>
So I think the below patch should fix that. But I have not tested it at all
and it could very well have issues. In particular, there is a likelihood of a
wake up while holding a lock which I'm not sure is safe due to scheduler
locks. I'll test it next week. Let me know any thoughts though.
---8<-----------------------
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: [PATCH] rcu: Fix race where wake_nocb_gp_defer() lazy wake can
overwrite a non-lazy wake
Fix by holding nocb_gp_lock when observing the state of all rdps. If any
rdp queued a non-lazy CB, we would do a wake up of the main gp thread.
This should address the race Frederick reported (which could effect both
non-lazy CBs using the bypass list, and lazy CBs, though lazy CBs much
more noticeably).
Quoting from Frederic's email:
1) rdp(1) has lazy callbacks
2) all other rdp's have no callback at all
3) nocb_gp_wait() runs through all rdp's, everything is handled, except for
these lazy callbacks
4) It reaches the above path, ready to arm the RCU_NOCB_WAKE_LAZY timer,
but it hasn't yet called wake_nocb_gp_defer()
5) Oh but rdp(2) queues a non-lazy callback. interrupts are disabled so it defers
the wake up to nocb_gp_wait() with arming the timer in RCU_NOCB_WAKE.
6) nocb_gp_wait() finally calls wake_nocb_gp_defer() and override the timeout
to several seconds ahead.
7) No more callbacks queued, the non-lazy callback will have to wait several
seconds to complete.
Here, the nocb gp lock is held when #4 happens. So the deferred wakeup
on #5 has to wait till #4 finishes.
Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
kernel/rcu/tree_nocb.h | 28 ++++++++++++++++++++++------
1 file changed, 22 insertions(+), 6 deletions(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 8b46442e4473..6690ece8fe20 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -285,14 +285,15 @@ EXPORT_SYMBOL(rcu_lazy_get_jiffies_till_flush);
* Arrange to wake the GP kthread for this NOCB group at some future
* time when it is safe to do so.
*/
-static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
- const char *reason)
+static void __wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
+ const char *reason, bool locked)
{
unsigned long flags;
struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
unsigned long mod_jif = 0;
- raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
+ if (!locked)
+ raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
/*
* Bypass and lazy wakeup overrides previous deferments. In case of
@@ -323,11 +324,23 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
if (rdp_gp->nocb_defer_wakeup < waketype)
WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
- raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
+ if (!locked)
+ raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
}
+static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
+ const char *reason) {
+ __wake_nocb_gp_defer(rdp, waketype, reason, false);
+}
+
+
+static void wake_nocb_gp_defer_locked(struct rcu_data *rdp, int waketype,
+ const char *reason) {
+ __wake_nocb_gp_defer(rdp, waketype, reason, true);
+
/*
* Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
* However, if there is a callback to be enqueued and if ->nocb_bypass
@@ -754,6 +767,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
* is added to the list, so the skipped-over rcu_data structures
* won't be ignored for long.
*/
+
+ raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
list_for_each_entry_rcu(rdp, &my_rdp->nocb_head_rdp, nocb_entry_rdp, 1) {
bool needwake_state = false;
bool flush_bypass = false;
@@ -855,14 +870,15 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
// If bypass list only has lazy CBs. Add a deferred
// lazy wake up.
if (lazy && !bypass) {
- wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_LAZY,
+ wake_nocb_gp_defer_locked(my_rdp, RCU_NOCB_WAKE_LAZY,
TPS("WakeLazyIsDeferred"));
// Otherwise add a deferred bypass wake up.
} else if (bypass) {
- wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_BYPASS,
+ wake_nocb_gp_defer_locked(my_rdp, RCU_NOCB_WAKE_BYPASS,
TPS("WakeBypassIsDeferred"));
}
}
+ raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
if (rcu_nocb_poll) {
/* Polling, so trace if first poll in the series. */
--
2.37.2.789.g6183377224-goog
next prev parent reply other threads:[~2022-09-03 22:00 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-01 22:17 [PATCH v5 00/18] Implement call_rcu_lazy() and miscellaneous fixes Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 01/18] mm/slub: perform free consistency checks before call_rcu Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head Joel Fernandes (Google)
2022-09-02 9:26 ` Vlastimil Babka
2022-09-02 9:30 ` Vlastimil Babka
2022-09-02 15:09 ` Joel Fernandes
2022-09-03 13:53 ` Paul E. McKenney
2022-09-01 22:17 ` [PATCH v5 03/18] rcu/tree: Use READ_ONCE() for lockless read of rnp->qsmask Joel Fernandes (Google)
2022-09-06 22:26 ` Boqun Feng
2022-09-06 22:31 ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass cblist happens Joel Fernandes (Google)
2022-09-02 11:35 ` Frederic Weisbecker
2022-09-02 23:58 ` Joel Fernandes
2022-09-03 15:10 ` Paul E. McKenney
2022-09-04 21:13 ` Frederic Weisbecker
2022-09-03 14:04 ` Paul E. McKenney
2022-09-03 14:05 ` Joel Fernandes
2022-09-06 3:07 ` Joel Fernandes
2022-09-06 9:48 ` Frederic Weisbecker
2022-09-07 2:43 ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 05/18] rcu: Move trace_rcu_callback() before bypassing Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation Joel Fernandes (Google)
2022-09-02 15:21 ` Frederic Weisbecker
2022-09-02 23:09 ` Joel Fernandes
2022-09-05 12:59 ` Frederic Weisbecker
2022-09-05 20:18 ` Joel Fernandes
2022-09-05 20:32 ` Joel Fernandes
2022-09-06 8:55 ` Paul E. McKenney
2022-09-06 16:16 ` Joel Fernandes
2022-09-06 17:05 ` Paul E. McKenney
2022-09-03 22:00 ` Joel Fernandes [this message]
2022-09-04 21:01 ` Frederic Weisbecker
2022-09-05 20:20 ` Joel Fernandes
2022-09-06 3:05 ` Joel Fernandes
2022-09-06 15:17 ` Frederic Weisbecker
2022-09-06 16:15 ` Joel Fernandes
2022-09-06 16:31 ` Joel Fernandes
2022-09-06 16:38 ` Joel Fernandes
2022-09-06 16:43 ` Joel Fernandes
2022-09-06 19:11 ` Frederic Weisbecker
2022-09-07 2:56 ` Joel Fernandes
2022-09-07 9:56 ` Frederic Weisbecker
2022-09-07 10:03 ` Frederic Weisbecker
2022-09-07 14:01 ` Joel Fernandes
2022-09-07 0:06 ` Joel Fernandes
2022-09-07 9:40 ` Frederic Weisbecker
2022-09-07 13:44 ` Joel Fernandes
2022-09-07 15:38 ` Frederic Weisbecker
2022-09-07 15:39 ` Joel Fernandes
2022-09-21 23:52 ` Joel Fernandes
2022-09-06 18:16 ` Uladzislau Rezki
2022-09-06 18:21 ` Joel Fernandes
2022-09-07 8:52 ` Uladzislau Rezki
2022-09-07 15:23 ` Joel Fernandes
2022-09-03 14:03 ` Paul E. McKenney
2022-09-03 14:05 ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 07/18] rcu: shrinker for lazy rcu Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 08/18] rcu: Add per-CB tracing for queuing, flush and invocation Joel Fernandes (Google)
2022-09-02 16:48 ` kernel test robot
2022-09-03 12:39 ` Paul E. McKenney
2022-09-03 14:07 ` Joel Fernandes
2022-09-02 19:01 ` kernel test robot
2022-09-01 22:17 ` [PATCH v5 09/18] rcuscale: Add laziness and kfree tests Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 10/18] rcutorture: Add test code for call_rcu_lazy() Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 11/18] fs: Move call_rcu() to call_rcu_lazy() in some paths Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 12/18] cred: Move call_rcu() to call_rcu_lazy() Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 13/18] security: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 14/18] net/core: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 15/18] kernel: Move various core kernel usages " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 16/18] lib: Move call_rcu() " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 17/18] i915: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 18/18] fork: Move thread_stack_free_rcu() " Joel Fernandes (Google)
2022-09-03 15:22 ` [PATCH v5 00/18] Implement call_rcu_lazy() and miscellaneous fixes Paul E. McKenney
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