Correct error line format of my mail content and add comments.
> -----邮件原件-----
> 发件人: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 发送时间: 2023年4月4日 11:23
> 收件人: 代子为 (Ziwei Dai) <Ziwei.Dai@xxxxxxxxxx>
> 抄送: urezki@xxxxxxxxx; frederic@xxxxxxxxxx; quic_neeraju@xxxxxxxxxxx; josh@xxxxxxxxxxxxxxxx; rostedt@xxxxxxxxxxx;
> mathieu.desnoyers@xxxxxxxxxxxx; jiangshanlai@xxxxxxxxx; joel@xxxxxxxxxxxxxxxxx; rcu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> 王双 (Shuang Wang) <shuang.wang@xxxxxxxxxx>; 辛依凡 (Yifan Xin) <Yifan.Xin@xxxxxxxxxx>; 王科 (Ke Wang)
> <Ke.Wang@xxxxxxxxxx>; 闫学文 (Xuewen Yan) <Xuewen.Yan@xxxxxxxxxx>; 牛志国 (Zhiguo Niu) <Zhiguo.Niu@xxxxxxxxxx>; 黄朝
> 阳 (Zhaoyang Huang) <zhaoyang.huang@xxxxxxxxxx>
> 主题: Re: 答复: [PATCH V2] rcu: Make sure new krcp free business is handled after the wanted rcu grace period.
>
>
>
> On Tue, Apr 04, 2023 at 02:49:15AM +0000, 代子为 (Ziwei Dai) wrote:
> > Hello Paul!
> >
> > > -----邮件原件-----
> > > 发件人: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > 发送时间: 2023年4月4日 6:58
> > > 收件人: 代子为 (Ziwei Dai) <Ziwei.Dai@xxxxxxxxxx>
> > > 抄送: urezki@xxxxxxxxx; frederic@xxxxxxxxxx; quic_neeraju@xxxxxxxxxxx;
> > > josh@xxxxxxxxxxxxxxxx; rostedt@xxxxxxxxxxx;
> > > mathieu.desnoyers@xxxxxxxxxxxx; jiangshanlai@xxxxxxxxx;
> > > joel@xxxxxxxxxxxxxxxxx; rcu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > 王双 (Shuang Wang) <shuang.wang@xxxxxxxxxx>; 辛依凡 (Yifan Xin)
> > > <Yifan.Xin@xxxxxxxxxx>; 王科 (Ke Wang) <Ke.Wang@xxxxxxxxxx>; 闫学文
> > > (Xuewen Yan) <Xuewen.Yan@xxxxxxxxxx>; 牛志国 (Zhiguo Niu)
> > > <Zhiguo.Niu@xxxxxxxxxx>; 黄朝阳 (Zhaoyang Huang)
> > > <zhaoyang.huang@xxxxxxxxxx>
> > > 主题: Re: [PATCH V2] rcu: Make sure new krcp free business is handled after
> > > the wanted rcu grace period.
> > >
> > >
> > > 注意: 这封邮件来自于外部。除非你确定邮件内容安全，否则不要点击任
> > > 何链接和附件。
> > > CAUTION: This email originated from outside of the organization. Do not click
> > > links or open attachments unless you recognize the sender and know the
> > > content is safe.
> > >
> > >
> > >
> > > On Fri, Mar 31, 2023 at 08:42:09PM +0800, Ziwei Dai wrote:
> > > > In kfree_rcu_monitor(), new free business at krcp is attached to any
> > > > free channel at krwp. kfree_rcu_monitor() is responsible to make sure
> > > > new free business is handled after the rcu grace period. But if there
> > > > is any none-free channel at krwp already, that means there is an
> > > > on-going rcu work, which will cause the kvfree_call_rcu()-triggered
> > > > free business is done before the wanted rcu grace period ends.
> > > >
> > > > This commit ignore krwp which has non-free channel at
> > > > kfree_rcu_monitor(), to fix the issue that kvfree_call_rcu() loses effectiveness.
> > > >
> > > > Below is the css_set obj "from_cset" use-after-free case caused by
> > > > kvfree_call_rcu() losing effectiveness.
> > > > CPU 0 calls rcu_read_lock(), then use "from_cset", then hard irq
> > > > comes, the task is schedule out.
> > > > CPU 1 calls kfree_rcu(cset, rcu_head), willing to free "from_cset" after new gp.
> > > > But "from_cset" is freed right after current gp end. "from_cset" is reallocated.
> > > > CPU 0 's task arrives back, references "from_cset"'s member, which causes crash.
> > > >
> > > > CPU 0 CPU 1
> > > > count_memcg_event_mm()
> > > > |rcu_read_lock() <---
> > > > |mem_cgroup_from_task()
> > > > |// css_set_ptr is the "from_cset" mentioned on CPU 1 |css_set_ptr =
> > > > rcu_dereference((task)->cgroups) |// Hard irq comes, current task is
> > > > scheduled out.
> > > >
> > > > cgroup_attach_task()
> > > > |cgroup_migrate()
> > > > |cgroup_migrate_execute()
> > > > |css_set_move_task(task, from_cset, to_cset, true)
> > > > |cgroup_move_task(task, to_cset)
> > > > |rcu_assign_pointer(.., to_cset)
> > > > |...
> > > > |cgroup_migrate_finish()
> > > > |put_css_set_locked(from_cset)
> > > > |from_cset->refcount return 0
> > > > |kfree_rcu(cset, rcu_head) // means to free from_cset after new gp
> > > > |add_ptr_to_bulk_krc_lock()
> > > > |schedule_delayed_work(&krcp->monitor_work, ..)
> > > >
> > > > kfree_rcu_monitor()
> > > > |krcp->bulk_head[0]'s work attached to krwp->bulk_head_free[]
> > > > |queue_rcu_work(system_wq, &krwp->rcu_work)
> > > > |if rwork->rcu.work is not in WORK_STRUCT_PENDING_BIT state,
> > > > |call_rcu(&rwork->rcu, rcu_work_rcufn) <--- request a new gp
> > > >
> > > > // There is a perious call_rcu(.., rcu_work_rcufn)
> > > > // gp end, rcu_work_rcufn() is called.
> > > > rcu_work_rcufn()
> > > > |__queue_work(.., rwork->wq, &rwork->work);
> > > >
> > > > |kfree_rcu_work()
> > > > |krwp->bulk_head_free[0] bulk is freed before new gp end!!!
> > > > |The "from_cset" is freed before new gp end.
> > > >
> > > > // the task is scheduled in after many ms.
> > > > |css_set_ptr->subsys[(subsys_id) <--- Caused kernel crash, because css_set_ptr is freed.
> > > >
> > > > v2: Use helper function instead of inserted code block at kfree_rcu_monitor().
> > > >
> > > > Fixes: c014efeef76a ("rcu: Add multiple in-flight batches of
> > > > kfree_rcu() work")
> > > > Signed-off-by: Ziwei Dai <ziwei.dai@xxxxxxxxxx>
> > >
> > > Good catch, thank you!!!
> > >
> > > How difficult was this to trigger? If it can be triggered easily, this of course
> > > needs to go into mainline sooner rather than later.
> >
> > Roughly we can reproduce this issue within two rounds of 48h stress test,
> > with 20 k5.15 devices. If KASAN is enabled, the reproduce rate is higher.
> > So I think sooner is better.
>
> Thank you for the info! This is in theory an old bug, but if you can
> easily find out, does it trigger for you on v6.2 or earlier?
>
We haven't ported v6.2 to our device yet...
> > > Longer term, would it make sense to run the three channels through RCU
> > > separately, in order to avoid one channel refraining from starting a grace
> > > period just because some other channel has callbacks waiting for a grace
> > > period to complete? One argument against might be energy efficiency, but
> > > perhaps the ->gp_snap field could be used to get the best of both worlds.
> >
> > I see kvfree_rcu_drain_ready(krcp) is already called at the beginning of
> > kfree_rcu_monitor(), which polls the ->gp_snap field, to decide
> > whether to free channel objects immediately or after gp.
> > Both energy efficiency and timing seems be considered?
>
> My concern is that running the channels separately might mean more grace
> periods (and thus more energy draw) on nearly idle devices, such devices
> usually being the ones for which energy efficiency matters most.
>
> But perhaps Vlad, Neeraj, or Joel has some insight on this, given
> that they are the ones working on battery-powered devices.
>
> > > Either way, this fixes only one bug of two. The second bug is in the
> > > kfree_rcu() tests, which should have caught this bug. Thoughts on a good fix
> > > for those tests?
> >
> > I inserted a msleep() between "rcu_read_lock(), get pointer via rcu_dereference()"
> > and "reference pointer, using the member", at the rcu scenario, then we can
> > reproduce this issue very soon in stress test. Can kfree_rcu() tests insert msleep()?
>
> Another approach is to separate concerns, so that readers interact with
> grace periods in the rcutorture.c tests, and to add the interaction
> of to-be-freed memory with grace periods in the rcuscale kvfree tests.
> I took a step in this direction with this commit on the -rcu tree's
> "dev" branch:
>
> efbe7927f479 ("rcu/kvfree: Add debug to check grace periods")
>
> Given this, might it be possible to make rcuscale.c's kfree_rcu()
> testing create patterns of usage of the three channels so as to
> catch this bug that way?
>
I can try it on my k5.15 device, and need some time.
I have a question. Do you mean add code in tree.c to create pattern
while channel data is being freed?
If so, both rcuscales.c and tree.c need to be modified for the test case.
> > > I have applied Uladzislau's and Mukesh's tags, and done the usual
> > > wordsmithing as shown at the end of this message. Please let me know if I
> > > messed anything up.
> >
> > Thank you for the improvement on the patch! It seems better now.
>
> No problem and thank you again for the debugging and the fix!
>
> Thanx, Paul
>
> > > > ---
> > > > kernel/rcu/tree.c | 27 +++++++++++++++++++--------
> > > > 1 file changed, 19 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index
> > > > 8e880c0..7b95ee9 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -3024,6 +3024,18 @@ static void kfree_rcu_work(struct work_struct *work)
> > > > return !!READ_ONCE(krcp->head);
> > > > }
> > > >
> > > > +static bool
> > > > +need_wait_for_krwp_work(struct kfree_rcu_cpu_work *krwp) {
> > > > + int i;
> > > > +
> > > > + for (i = 0; i < FREE_N_CHANNELS; i++)
> > > > + if (!list_empty(&krwp->bulk_head_free[i]))
> > > > + return true;
> > > > +
> > > > + return !!krwp->head_free;
> > >
> > > This is fixed from v1, good!
> > >
> > > > +}
> > > > +
> > > > static int krc_count(struct kfree_rcu_cpu *krcp) {
> > > > int sum = atomic_read(&krcp->head_count); @@ -3107,15 +3119,14
> > > > @@ static void kfree_rcu_monitor(struct work_struct *work)
> > > > for (i = 0; i < KFREE_N_BATCHES; i++) {
> > > > struct kfree_rcu_cpu_work *krwp = &(krcp->krw_arr[i]);
> > > >
> > > > - // Try to detach bulk_head or head and attach it over any
> > > > - // available corresponding free channel. It can be that
> > > > - // a previous RCU batch is in progress, it means that
> > > > - // immediately to queue another one is not possible so
> > > > - // in that case the monitor work is rearmed.
> > > > - if ((!list_empty(&krcp->bulk_head[0]) && list_empty(&krwp->bulk_head_free[0])) ||
> > > > - (!list_empty(&krcp->bulk_head[1]) && list_empty(&krwp->bulk_head_free[1])) ||
> > > > - (READ_ONCE(krcp->head) && !krwp->head_free)) {
> > > > + // Try to detach bulk_head or head and attach it, only when
> > > > + // all channels are free. Any channel is not free means at krwp
> > > > + // there is on-going rcu work to handle krwp's free business.
> > > > + if (need_wait_for_krwp_work(krwp))
> > > > + continue;
> > > >
> > > > + // kvfree_rcu_drain_ready() might handle this krcp, if so give up.
> > > > + if (need_offload_krc(krcp)) {
> > > > // Channel 1 corresponds to the SLAB-pointer bulk path.
> > > > // Channel 2 corresponds to vmalloc-pointer bulk path.
> > > > for (j = 0; j < FREE_N_CHANNELS; j++) {
> > > > --
> > > > 1.9.1
> > >
> > > ------------------------------------------------------------------------
> > >
> > > commit e222f9a512539c3f4093a55d16624d9da614800b
> > > Author: Ziwei Dai <ziwei.dai@xxxxxxxxxx>
> > > Date: Fri Mar 31 20:42:09 2023 +0800
> > >
> > > rcu: Avoid freeing new kfree_rcu() memory after old grace period
> > >
> > > Memory passed to kvfree_rcu() that is to be freed is tracked by a
> > > per-CPU kfree_rcu_cpu structure, which in turn contains pointers
> > > to kvfree_rcu_bulk_data structures that contain pointers to memory
> > > that has not yet been handed to RCU, along with an kfree_rcu_cpu_work
> > > structure that tracks the memory that has already been handed to RCU.
> > > These structures track three categories of memory: (1) Memory for
> > > kfree(), (2) Memory for kvfree(), and (3) Memory for both that arrived
> > > during an OOM episode. The first two categories are tracked in a
> > > cache-friendly manner involving a dynamically allocated page of pointers
> > > (the aforementioned kvfree_rcu_bulk_data structures), while the third
> > > uses a simple (but decidedly cache-unfriendly) linked list through the
> > > rcu_head structures in each block of memory.
> > >
> > > On a given CPU, these three categories are handled as a unit, with that
> > > CPU's kfree_rcu_cpu_work structure having one pointer for each of the
> > > three categories. Clearly, new memory for a given category cannot be
> > > placed in the corresponding kfree_rcu_cpu_work structure until any old
> > > memory has had its grace period elapse and thus has been removed. And
> > > the kfree_rcu_monitor() function does in fact check for this.
> > >
> > > Except that the kfree_rcu_monitor() function checks these pointers one
> > > at a time. This means that if the previous kfree_rcu() memory passed
> > > to RCU had only category 1 and the current one has only category 2, the
> > > kfree_rcu_monitor() function will send that current category-2 memory
> > > along immediately. This can result in memory being freed too soon,
> > > that is, out from under unsuspecting RCU readers.
> > >
> > > To see this, consider the following sequence of events, in which:
> > >
> > > o Task A on CPU 0 calls rcu_read_lock(), then uses "from_cset",
> > > then is preempted.
> > >
> > > o CPU 1 calls kfree_rcu(cset, rcu_head) in order to free "from_cset"
> > > after a later grace period. Except that "from_cset" is freed
> > > right after the previous grace period ended, so that "from_cset"
> > > is immediately freed. Task A resumes and references "from_cset"'s
> > > member, after which nothing good happens.
> > >
> > > In full detail:
> > >
> > > CPU 0 CPU 1
> > > ---------------------- ----------------------
> > > count_memcg_event_mm()
> > > |rcu_read_lock() <---
> > > |mem_cgroup_from_task()
> > > |// css_set_ptr is the "from_cset" mentioned on CPU 1
> > > |css_set_ptr = rcu_dereference((task)->cgroups)
> > > |// Hard irq comes, current task is scheduled out.
> > >
> > > cgroup_attach_task()
> > > |cgroup_migrate()
> > > |cgroup_migrate_execute()
> > > |css_set_move_task(task, from_cset, to_cset, true)
> > > |cgroup_move_task(task, to_cset)
> > > |rcu_assign_pointer(.., to_cset)
> > > |...
> > > |cgroup_migrate_finish()
> > > |put_css_set_locked(from_cset)
> > > |from_cset->refcount return 0
> > > |kfree_rcu(cset, rcu_head) // free from_cset after new gp
> > > |add_ptr_to_bulk_krc_lock()
> > > |schedule_delayed_work(&krcp->monitor_work, ..)
> > >
> > > kfree_rcu_monitor()
> > > |krcp->bulk_head[0]'s work attached to krwp->bulk_head_free[]
> > > |queue_rcu_work(system_wq, &krwp->rcu_work)
> > > |if rwork->rcu.work is not in WORK_STRUCT_PENDING_BIT state,
> > > |call_rcu(&rwork->rcu, rcu_work_rcufn) <--- request new gp
> > >
> > > // There is a perious call_rcu(.., rcu_work_rcufn)
> > > // gp end, rcu_work_rcufn() is called.
> > > rcu_work_rcufn()
> > > |__queue_work(.., rwork->wq, &rwork->work);
> > >
> > > |kfree_rcu_work()
> > > |krwp->bulk_head_free[0] bulk is freed before new gp end!!!
> > > |The "from_cset" is freed before new gp end.
> > >
> > > // the task resumes some time later.
> > > |css_set_ptr->subsys[(subsys_id) <--- Caused kernel crash, because css_set_ptr is freed.
> > >
> > > This commit therefore causes kfree_rcu_monitor() to refrain from moving
> > > kfree_rcu() memory to the kfree_rcu_cpu_work structure until the RCU
> > > grace period has completed for all three categories.
> > >
> > > v2: Use helper function instead of inserted code block at kfree_rcu_monitor().
> > >
> > > Fixes: c014efeef76a ("rcu: Add multiple in-flight batches of kfree_rcu() work")
> > > Reported-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> > > Signed-off-by: Ziwei Dai <ziwei.dai@xxxxxxxxxx>
> > > Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index
> > > 859ee02f6614..e2dbea6cee4b 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -3051,6 +3051,18 @@ need_offload_krc(struct kfree_rcu_cpu *krcp)
> > > return !!READ_ONCE(krcp->head);
> > > }
> > >
> > > +static bool
> > > +need_wait_for_krwp_work(struct kfree_rcu_cpu_work *krwp) {
> > > + int i;
> > > +
> > > + for (i = 0; i < FREE_N_CHANNELS; i++)
> > > + if (!list_empty(&krwp->bulk_head_free[i]))
> > > + return true;
> > > +
> > > + return !!krwp->head_free;
> > > +}
> > > +
> > > static int krc_count(struct kfree_rcu_cpu *krcp) {
> > > int sum = atomic_read(&krcp->head_count); @@ -3134,15
> > > +3146,14 @@ static void kfree_rcu_monitor(struct work_struct *work)
> > > for (i = 0; i < KFREE_N_BATCHES; i++) {
> > > struct kfree_rcu_cpu_work *krwp = &(krcp->krw_arr[i]);
> > >
> > > - // Try to detach bulk_head or head and attach it over any
> > > - // available corresponding free channel. It can be that
> > > - // a previous RCU batch is in progress, it means that
> > > - // immediately to queue another one is not possible so
> > > - // in that case the monitor work is rearmed.
> > > - if ((!list_empty(&krcp->bulk_head[0]) && list_empty(&krwp->bulk_head_free[0])) ||
> > > - (!list_empty(&krcp->bulk_head[1]) && list_empty(&krwp->bulk_head_free[1])) ||
> > > - (READ_ONCE(krcp->head) && !krwp->head_free)) {
> > > + // Try to detach bulk_head or head and attach it, only when
> > > + // all channels are free. Any channel is not free means at krwp
> > > + // there is on-going rcu work to handle krwp's free business.
> > > + if (need_wait_for_krwp_work(krwp))
> > > + continue;
> > >
> > > + // kvfree_rcu_drain_ready() might handle this krcp, if so give up.
> > > + if (need_offload_krc(krcp)) {
> > > // Channel 1 corresponds to the SLAB-pointer bulk path.
> > > // Channel 2 corresponds to vmalloc-pointer bulk path.
> > > for (j = 0; j < FREE_N_CHANNELS; j++) {
