Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping
locks. Therefore, it can't be called from an non-preemptible context.
One practical example is splat inside inactive_task_timer(), which is
called in a interrupt context:
CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W ---------
Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012
Call Trace:
dump_stack_lvl+0x57/0x7d
mark_lock_irq.cold+0x33/0xba
? stack_trace_save+0x4b/0x70
? save_trace+0x55/0x150
mark_lock+0x1e7/0x400
mark_usage+0x11d/0x140
__lock_acquire+0x30d/0x930
lock_acquire.part.0+0x9c/0x210
? refill_obj_stock+0x3d/0x3a0
? rcu_read_lock_sched_held+0x3f/0x70
? trace_lock_acquire+0x38/0x140
? lock_acquire+0x30/0x80
? refill_obj_stock+0x3d/0x3a0
rt_spin_lock+0x27/0xe0
? refill_obj_stock+0x3d/0x3a0
refill_obj_stock+0x3d/0x3a0
? inactive_task_timer+0x1ad/0x340
kmem_cache_free+0x357/0x560
inactive_task_timer+0x1ad/0x340
? switched_from_dl+0x2d0/0x2d0
__run_hrtimer+0x8a/0x1a0
__hrtimer_run_queues+0x91/0x130
hrtimer_interrupt+0x10f/0x220
__sysvec_apic_timer_interrupt+0x7b/0xd0
sysvec_apic_timer_interrupt+0x4f/0xd0
? asm_sysvec_apic_timer_interrupt+0xa/0x20
asm_sysvec_apic_timer_interrupt+0x12/0x20
RIP: 0033:0x7fff196bf6f5
Instead of calling __put_task_struct() directly, we defer it using
call_rcu(). A more natural approach would use a workqueue, but since
in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
the code would become more complex because we would need to put the
work_struct instance in the task_struct and initialize it when we
allocate a new task_struct.
Changelog
=========
v1:
* Initial implementation fixing the splat.
v2:
* Isolate the logic in its own function.
* Fix two more cases caught in review.
v3:
* Change __put_task_struct() to handle the issue internally.
v4:
* Explain why call_rcu() is safe to call from interrupt context.
v5:
* Explain why __put_task_struct() doesn't conflict with
put_task_sruct_rcu_user.
v6:
* As per Sebastian's review, revert back the implementation of v2
with a distinct function.
* Add a check in put_task_struct() to warning when called from a
non-sleepable context.
* Address more call sites.
v7:
* Fix typos.
* Add an explanation why the new function doesn't conflict with
delayed_free_task().
v8:
* Bring back v5.
* Fix coding style.
v9:
* Reorganize to not need ___put_task_struct() by Oleg's suggestion.
Signed-off-by: Wander Lairson Costa <[email protected]>
Reported-by: Hu Chunyu <[email protected]>
Suggested-by: Oleg Nesterov <[email protected]>
Suggested-by: Valentin Schneider <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Paul McKenney <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Luis Goncalves <[email protected]>
---
include/linux/sched/task.h | 28 +++++++++++++++++++++++++++-
kernel/fork.c | 8 ++++++++
2 files changed, 35 insertions(+), 1 deletion(-)
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index d6c48163c6de..9bcb9535d4e1 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -112,10 +112,36 @@ static inline struct task_struct *get_task_struct(struct task_struct *t)
}
extern void __put_task_struct(struct task_struct *t);
+extern void __put_task_struct_rcu_cb(struct rcu_head *rhp);
static inline void put_task_struct(struct task_struct *t)
{
- if (refcount_dec_and_test(&t->usage))
+ if (!refcount_dec_and_test(&t->usage))
+ return;
+
+ /*
+ * under PREEMPT_RT, we can't call put_task_struct
+ * in atomic context because it will indirectly
+ * acquire sleeping locks.
+ *
+ * call_rcu() will schedule delayed_put_task_struct_rcu()
+ * to be called in process context.
+ *
+ * __put_task_struct() is called when
+ * refcount_dec_and_test(&t->usage) succeeds.
+ *
+ * This means that it can't "conflict" with
+ * put_task_struct_rcu_user() which abuses ->rcu the same
+ * way; rcu_users has a reference so task->usage can't be
+ * zero after rcu_users 1 -> 0 transition.
+ *
+ * delayed_free_task() also uses ->rcu, but it is only called
+ * when it fails to fork a process. Therefore, there is no
+ * way it can conflict with put_task_struct().
+ */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
+ call_rcu(&t->rcu, __put_task_struct_rcu_cb);
+ else
__put_task_struct(t);
}
diff --git a/kernel/fork.c b/kernel/fork.c
index 08969f5aa38d..fd3bb4a554c4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -846,6 +846,14 @@ void __put_task_struct(struct task_struct *tsk)
}
EXPORT_SYMBOL_GPL(__put_task_struct);
+void __put_task_struct_rcu_cb(struct rcu_head *rhp)
+{
+ struct task_struct *task = container_of(rhp, struct task_struct, rcu);
+
+ __put_task_struct(task);
+}
+EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb);
+
void __init __weak arch_task_cache_init(void) { }
/*
--
2.40.1
On Tue, May 16, 2023 at 04:14:41PM -0300, Wander Lairson Costa wrote:
> +void __put_task_struct_rcu_cb(struct rcu_head *rhp)
> +{
> + struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> +
> + __put_task_struct(task);
> +}
> +EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb);
Why does this need to be exported when its only caller is within the
main kernel and cannot possibly be built as a module?
On Tue, 16 May 2023 20:24:04 +0100 Matthew Wilcox <[email protected]> wrote:
> On Tue, May 16, 2023 at 04:14:41PM -0300, Wander Lairson Costa wrote:
> > +void __put_task_struct_rcu_cb(struct rcu_head *rhp)
> > +{
> > + struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > +
> > + __put_task_struct(task);
> > +}
> > +EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb);
>
> Why does this need to be exported when its only caller is within the
> main kernel and cannot possibly be built as a module?
It's referenced by inlined put_task_struct(), which is called from all
over.
However I believe the above definition could be inside #ifdef
CONFIG_PREEMPT_RT, to save a scrap of resources?
On Tue, May 16, 2023 at 02:05:55PM -0700, Andrew Morton wrote:
> On Tue, 16 May 2023 20:24:04 +0100 Matthew Wilcox <[email protected]> wrote:
>
> > On Tue, May 16, 2023 at 04:14:41PM -0300, Wander Lairson Costa wrote:
> > > +void __put_task_struct_rcu_cb(struct rcu_head *rhp)
> > > +{
> > > + struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > > +
> > > + __put_task_struct(task);
> > > +}
> > > +EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb);
> >
> > Why does this need to be exported when its only caller is within the
> > main kernel and cannot possibly be built as a module?
>
> It's referenced by inlined put_task_struct(), which is called from all
> over.
Oh, I missed that put_task_struct() was still inlined. Should it be?
It seems quite large now.
> However I believe the above definition could be inside #ifdef
> CONFIG_PREEMPT_RT, to save a scrap of resources?
On Tue, 16 May 2023 22:41:18 +0100 Matthew Wilcox <[email protected]> wrote:
>
> Oh, I missed that put_task_struct() was still inlined. Should it be?
> It seems quite large now.
It's not significantly worse because of this patch. In fact, it's
unchanged for non-RT kernels.
Possibly put_task_struct() *should* be uninlined, because it made the
mistake of using the dang refcount stuff, which never saw a byte which
it couldn't consume :(
I mean...
--- a/fs/open.c~a
+++ a/fs/open.c
@@ -1572,3 +1572,9 @@ int stream_open(struct inode *inode, str
}
EXPORT_SYMBOL(stream_open);
+
+#include <linux/refcount.h>
+bool foo(refcount_t *r)
+{
+ return refcount_dec_and_test(r);
+}
_
is worth
339 bytes of text for an allmodconfig build
67 bytes of text for an allnoconfig build
77 bytes of text for a defconfig build
On 05/16, Wander Lairson Costa wrote:
>
> static inline void put_task_struct(struct task_struct *t)
> {
> - if (refcount_dec_and_test(&t->usage))
> + if (!refcount_dec_and_test(&t->usage))
> + return;
> +
> + /*
> + * under PREEMPT_RT, we can't call put_task_struct
> + * in atomic context because it will indirectly
> + * acquire sleeping locks.
> + *
> + * call_rcu() will schedule delayed_put_task_struct_rcu()
> + * to be called in process context.
> + *
> + * __put_task_struct() is called when
> + * refcount_dec_and_test(&t->usage) succeeds.
> + *
> + * This means that it can't "conflict" with
> + * put_task_struct_rcu_user() which abuses ->rcu the same
> + * way; rcu_users has a reference so task->usage can't be
> + * zero after rcu_users 1 -> 0 transition.
> + *
> + * delayed_free_task() also uses ->rcu, but it is only called
> + * when it fails to fork a process. Therefore, there is no
> + * way it can conflict with put_task_struct().
> + */
> + if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> + call_rcu(&t->rcu, __put_task_struct_rcu_cb);
> + else
> __put_task_struct(t);
> }
LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
Again, I'll try to investigate when I have time although I am not sure I can really help.
Perhaps you too can try to do this ? ;)
Oleg.
On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <[email protected]> wrote:
>
> On 05/16, Wander Lairson Costa wrote:
> >
> > static inline void put_task_struct(struct task_struct *t)
> > {
> > - if (refcount_dec_and_test(&t->usage))
> > + if (!refcount_dec_and_test(&t->usage))
> > + return;
> > +
> > + /*
> > + * under PREEMPT_RT, we can't call put_task_struct
> > + * in atomic context because it will indirectly
> > + * acquire sleeping locks.
> > + *
> > + * call_rcu() will schedule delayed_put_task_struct_rcu()
> > + * to be called in process context.
> > + *
> > + * __put_task_struct() is called when
> > + * refcount_dec_and_test(&t->usage) succeeds.
> > + *
> > + * This means that it can't "conflict" with
> > + * put_task_struct_rcu_user() which abuses ->rcu the same
> > + * way; rcu_users has a reference so task->usage can't be
> > + * zero after rcu_users 1 -> 0 transition.
> > + *
> > + * delayed_free_task() also uses ->rcu, but it is only called
> > + * when it fails to fork a process. Therefore, there is no
> > + * way it can conflict with put_task_struct().
> > + */
> > + if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > + call_rcu(&t->rcu, __put_task_struct_rcu_cb);
> > + else
> > __put_task_struct(t);
> > }
>
> LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
>
> Again, I'll try to investigate when I have time although I am not sure I can really help.
>
> Perhaps you too can try to do this ? ;)
>
FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
stock kernels. No splat happened.
> Oleg.
>
On 05/17, Wander Lairson Costa wrote:
>
> On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <[email protected]> wrote:
> >
> > LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
> >
> > Again, I'll try to investigate when I have time although I am not sure I can really help.
> >
> > Perhaps you too can try to do this ? ;)
> >
>
> FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
> stock kernels. No splat happened.
Strange... FYI, I am running the kernel with this patch
diff --git a/kernel/sys.c b/kernel/sys.c
index 339fee3eff6a..3169cceddf3b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2412,6 +2412,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
error = 0;
switch (option) {
+ case 666: {
+ static DEFINE_SPINLOCK(l);
+ static DEFINE_RAW_SPINLOCK(r);
+
+ raw_spin_lock(&r);
+ spin_lock(&l);
+ spin_unlock(&l);
+ raw_spin_unlock(&r);
+
+ break;
+ }
case PR_SET_PDEATHSIG:
if (!valid_signal(arg2)) {
error = -EINVAL;
applied (because I am too lazy to compile a module ;) and
# perl -e 'syscall 157,666'
triggers the lockdep bug
=============================
[ BUG: Invalid wait context ]
6.4.0-rc2-00018-g4d6d4c7f541d-dirty #1176 Not tainted
-----------------------------
perl/35 is trying to lock:
ffffffff81c4cc18 (l){....}-{3:3}, at: __do_sys_prctl+0x21b/0x87b
other info that might help us debug this:
context-{5:5}
...
as expected.
Looks like your testing was wrong... Or maybe you missed another lockdep problem ?
Did you check dmesg? Perhaps lockdep detected another bug,say, even at boot time ?
In this case debug_locks_off() sets debug_locks = 0 and this disables lockdep.
Oleg.
On Mon, May 29, 2023 at 9:23 AM Oleg Nesterov <[email protected]> wrote:
>
> On 05/17, Wander Lairson Costa wrote:
> >
> > On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <[email protected]> wrote:
> > >
> > > LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
> > >
> > > Again, I'll try to investigate when I have time although I am not sure I can really help.
> > >
> > > Perhaps you too can try to do this ? ;)
> > >
> >
> > FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
> > stock kernels. No splat happened.
>
> Strange... FYI, I am running the kernel with this patch
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 339fee3eff6a..3169cceddf3b 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2412,6 +2412,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>
> error = 0;
> switch (option) {
> + case 666: {
> + static DEFINE_SPINLOCK(l);
> + static DEFINE_RAW_SPINLOCK(r);
> +
> + raw_spin_lock(&r);
> + spin_lock(&l);
> + spin_unlock(&l);
> + raw_spin_unlock(&r);
> +
> + break;
> + }
> case PR_SET_PDEATHSIG:
> if (!valid_signal(arg2)) {
> error = -EINVAL;
>
> applied (because I am too lazy to compile a module ;) and
>
FWIW, I converted it to a module [1]
> # perl -e 'syscall 157,666'
>
> triggers the lockdep bug
>
> =============================
> [ BUG: Invalid wait context ]
> 6.4.0-rc2-00018-g4d6d4c7f541d-dirty #1176 Not tainted
> -----------------------------
> perl/35 is trying to lock:
> ffffffff81c4cc18 (l){....}-{3:3}, at: __do_sys_prctl+0x21b/0x87b
> other info that might help us debug this:
> context-{5:5}
> ...
>
> as expected.
>
Yeah, I tried it here and I had the same results, but only in the RT
kernel. But running the reproducer for put_task_struct(), works fine.
> Looks like your testing was wrong... Or maybe you missed another lockdep problem ?
> Did you check dmesg? Perhaps lockdep detected another bug,say, even at boot time ?
> In this case debug_locks_off() sets debug_locks = 0 and this disables lockdep.
>
> Oleg.
>
On 06/01, Wander Lairson Costa wrote:
>
> On Mon, May 29, 2023 at 9:23 AM Oleg Nesterov <[email protected]> wrote:
> >
> > On 05/17, Wander Lairson Costa wrote:
> > >
> > > On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <[email protected]> wrote:
> > > >
> > > > LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
> > > >
> > > > Again, I'll try to investigate when I have time although I am not sure I can really help.
> > > >
> > > > Perhaps you too can try to do this ? ;)
> > > >
> > >
> > > FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
> > > stock kernels. No splat happened.
> >
> > Strange... FYI, I am running the kernel with this patch
> >
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 339fee3eff6a..3169cceddf3b 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -2412,6 +2412,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> >
> > error = 0;
> > switch (option) {
> > + case 666: {
> > + static DEFINE_SPINLOCK(l);
> > + static DEFINE_RAW_SPINLOCK(r);
> > +
> > + raw_spin_lock(&r);
> > + spin_lock(&l);
> > + spin_unlock(&l);
> > + raw_spin_unlock(&r);
> > +
> > + break;
> > + }
> > case PR_SET_PDEATHSIG:
> > if (!valid_signal(arg2)) {
> > error = -EINVAL;
> >
> > applied (because I am too lazy to compile a module ;) and
> >
>
> FWIW, I converted it to a module [1]
where is [1] ? not that I think this matters though...
> > # perl -e 'syscall 157,666'
> >
> > triggers the lockdep bug
> >
> > =============================
> > [ BUG: Invalid wait context ]
> > 6.4.0-rc2-00018-g4d6d4c7f541d-dirty #1176 Not tainted
> > -----------------------------
> > perl/35 is trying to lock:
> > ffffffff81c4cc18 (l){....}-{3:3}, at: __do_sys_prctl+0x21b/0x87b
> > other info that might help us debug this:
> > context-{5:5}
> > ...
> >
> > as expected.
> >
>
> Yeah, I tried it here and I had the same results,
OK,
> but only in the RT kernel
this again suggests that your testing was wrong or I am totally confused (quite
possible, I know nothing about RT). I did the testing without CONFIG_PREEMPT_RT.
> But running the reproducer for put_task_struct(), works fine.
which reproducer ?
Oleg.
On Thu, Jun 1, 2023 at 3:14 PM Oleg Nesterov <[email protected]> wrote:
>
> On 06/01, Wander Lairson Costa wrote:
> >
> > On Mon, May 29, 2023 at 9:23 AM Oleg Nesterov <[email protected]> wrote:
> > >
> > > On 05/17, Wander Lairson Costa wrote:
> > > >
> > > > On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <[email protected]> wrote:
> > > > >
> > > > > LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
> > > > >
> > > > > Again, I'll try to investigate when I have time although I am not sure I can really help.
> > > > >
> > > > > Perhaps you too can try to do this ? ;)
> > > > >
> > > >
> > > > FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
> > > > stock kernels. No splat happened.
> > >
> > > Strange... FYI, I am running the kernel with this patch
> > >
> > > diff --git a/kernel/sys.c b/kernel/sys.c
> > > index 339fee3eff6a..3169cceddf3b 100644
> > > --- a/kernel/sys.c
> > > +++ b/kernel/sys.c
> > > @@ -2412,6 +2412,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> > >
> > > error = 0;
> > > switch (option) {
> > > + case 666: {
> > > + static DEFINE_SPINLOCK(l);
> > > + static DEFINE_RAW_SPINLOCK(r);
> > > +
> > > + raw_spin_lock(&r);
> > > + spin_lock(&l);
> > > + spin_unlock(&l);
> > > + raw_spin_unlock(&r);
> > > +
> > > + break;
> > > + }
> > > case PR_SET_PDEATHSIG:
> > > if (!valid_signal(arg2)) {
> > > error = -EINVAL;
> > >
> > > applied (because I am too lazy to compile a module ;) and
> > >
> >
> > FWIW, I converted it to a module [1]
>
> where is [1] ? not that I think this matters though...
>
> > > # perl -e 'syscall 157,666'
> > >
> > > triggers the lockdep bug
> > >
> > > =============================
> > > [ BUG: Invalid wait context ]
> > > 6.4.0-rc2-00018-g4d6d4c7f541d-dirty #1176 Not tainted
> > > -----------------------------
> > > perl/35 is trying to lock:
> > > ffffffff81c4cc18 (l){....}-{3:3}, at: __do_sys_prctl+0x21b/0x87b
> > > other info that might help us debug this:
> > > context-{5:5}
> > > ...
> > >
> > > as expected.
> > >
> >
> > Yeah, I tried it here and I had the same results,
>
> OK,
>
> > but only in the RT kernel
>
> this again suggests that your testing was wrong or I am totally confused (quite
> possible, I know nothing about RT). I did the testing without CONFIG_PREEMPT_RT.
>
Hrm, could you please share your .config?
> > But running the reproducer for put_task_struct(), works fine.
>
> which reproducer ?
>
Only now I noticed I didn't add the reproducer to the commit message:
while true; do
stress-ng --sched deadline --sched-period 1000000000
--sched-runtime 800000000 --sched-deadline 1000000000 --mmapfork 23 -t
20
done
On 06/01, Wander Lairson Costa wrote:
>
> On Thu, Jun 1, 2023 at 3:14 PM Oleg Nesterov <[email protected]> wrote:
> >
> > > but only in the RT kernel
> >
> > this again suggests that your testing was wrong or I am totally confused (quite
> > possible, I know nothing about RT). I did the testing without CONFIG_PREEMPT_RT.
> >
>
> Hrm, could you please share your .config?
Sure. I do not want to spam the list, I'll send you a private email.
Can you share your kernel module code?
Did you verify that debug_locks != 0 as I asked in my previous email ?
> > > But running the reproducer for put_task_struct(), works fine.
> >
> > which reproducer ?
> >
>
> Only now I noticed I didn't add the reproducer to the commit message:
>
> while true; do
> stress-ng --sched deadline --sched-period 1000000000
> --sched-runtime 800000000 --sched-deadline 1000000000 --mmapfork 23 -t
> 20
> done
Cough ;) I think we need a more simple one to enssure that
refcount_sub_and_test(nr, &t->usage) returns true under raw_spin_lock()
and then __put_task_struct() actually takes spin_lock().
Oleg.
On 06/01, Wander Lairson Costa wrote:
>
> On Thu, Jun 1, 2023 at 3:14 PM Oleg Nesterov <[email protected]> wrote:
> >
> > > but only in the RT kernel
> >
> > this again suggests that your testing was wrong or I am totally confused (quite
> > possible, I know nothing about RT). I did the testing without CONFIG_PREEMPT_RT.
> >
>
> Hrm, could you please share your .config?
Sure. I do not want to spam the list, I'll send you a private email.
Can you share your kernel module code?
Did you verify that debug_locks != 0 as I asked in my previous email ?
> > > But running the reproducer for put_task_struct(), works fine.
> >
> > which reproducer ?
> >
>
> Only now I noticed I didn't add the reproducer to the commit message:
>
> while true; do
> stress-ng --sched deadline --sched-period 1000000000
> --sched-runtime 800000000 --sched-deadline 1000000000 --mmapfork 23 -t
> 20
> done
Cough ;) I think we need something more simple to ensure that
refcount_sub_and_test(nr, &t->usage) returns true under raw_spin_lock()
and then __put_task_struct() actually takes spin_lock().
Oleg.
On Fri, Jun 2, 2023 at 2:34 PM Oleg Nesterov <[email protected]> wrote:
>
> On 06/01, Wander Lairson Costa wrote:
> >
> > On Thu, Jun 1, 2023 at 3:14 PM Oleg Nesterov <[email protected]> wrote:
> > >
> > > > but only in the RT kernel
> > >
> > > this again suggests that your testing was wrong or I am totally confused (quite
> > > possible, I know nothing about RT). I did the testing without CONFIG_PREEMPT_RT.
> > >
> >
> > Hrm, could you please share your .config?
>
> Sure. I do not want to spam the list, I'll send you a private email.
>
Thanks. I found an unrelated earlier splat in the console code. That's
why I couldn't reproduce it in the stock kernel.
> Can you share your kernel module code?
>
*facepalm* I forgot to post the link: https://github.com/walac/test-prove-lock/
> Did you verify that debug_locks != 0 as I asked in my previous email ?
>
> > > > But running the reproducer for put_task_struct(), works fine.
> > >
> > > which reproducer ?
> > >
> >
> > Only now I noticed I didn't add the reproducer to the commit message:
> >
> > while true; do
> > stress-ng --sched deadline --sched-period 1000000000
> > --sched-runtime 800000000 --sched-deadline 1000000000 --mmapfork 23 -t
> > 20
> > done
>
> Cough ;) I think we need a more simple one to enssure that
> refcount_sub_and_test(nr, &t->usage) returns true under raw_spin_lock()
> and then __put_task_struct() actually takes spin_lock().
>
> Oleg.
>
On 06/05, Wander Lairson Costa wrote:
>
> Thanks. I found an unrelated earlier splat in the console code. That's
> why I couldn't reproduce it in the stock kernel.
As expected...
So... Not sure what can I say ;) can you verify that this patch doesn't solve
the issues with CONFIG_PROVE_RAW_LOCK_NESTING pointed out by Sebastian? Using
stress-ng or anything else.
This is not that bad, unless I am totally confused the current code (without
your patch) has the same problem (otherwise we wouldn't need this fix).
But perhaps you can make 2/2 which adds the DEFINE_WAIT_OVERRIDE_MAP() hack
as Peter suggested?
Oleg.
On Tue, Jun 6, 2023 at 5:40 PM Oleg Nesterov <[email protected]> wrote:
>
> On 06/05, Wander Lairson Costa wrote:
> >
> > Thanks. I found an unrelated earlier splat in the console code. That's
> > why I couldn't reproduce it in the stock kernel.
>
> As expected...
>
> So... Not sure what can I say ;) can you verify that this patch doesn't solve
> the issues with CONFIG_PROVE_RAW_LOCK_NESTING pointed out by Sebastian? Using
> stress-ng or anything else.
>
I managed to test it without a console. No issues happened in the stock kernel.
> This is not that bad, unless I am totally confused the current code (without
> your patch) has the same problem (otherwise we wouldn't need this fix).
>
That's my understanding as well.
> But perhaps you can make 2/2 which adds the DEFINE_WAIT_OVERRIDE_MAP() hack
> as Peter suggested?
>
Yes, sure. I would like to get the issue reproduced in practice to
make sure I am really fixing the problem. But I can live with that.
> Oleg.
>