include/linux/sched/task.h | 21 +++++++++++++++++++++ kernel/fork.c | 8 ++++++++ kernel/sched/core.c | 2 +- kernel/sched/deadline.c | 2 +- kernel/sched/rt.c | 4 ++-- 5 files changed, 33 insertions(+), 4 deletions(-)
put_task_struct() decrements a usage counter and calls __put_task_struct() if the counter reaches zero. __put_task_struct() indirectly acquires a spinlock, which is a sleeping lock under PREEMPT_RT. Therefore, we can't call put_task_struct() in an atomic context in RT kernels. This patch series introduces put_task_struct_atomic_safe(), which defers the call to __put_task_struct() to a process context when compiled with PREEMPT_RT. It also fixes known problematic call sites. Changelog: ========== v2: * Add the put_task_struct_atomic_safe() function that is responsible for handling the conditions to call put_task_struct(). * Replace put_task_struct() by put_task_struct_atomic_safe() in known atomic call sites. Wander Lairson Costa (4): sched/task: Add the put_task_struct_atomic_safe function sched/deadline: fix inactive_task_timer splat sched/rt: use put_task_struct_atomic_safe() to avoid potential splat sched/core: use put_task_struct_atomic_safe() to avoid potential splat include/linux/sched/task.h | 21 +++++++++++++++++++++ kernel/fork.c | 8 ++++++++ kernel/sched/core.c | 2 +- kernel/sched/deadline.c | 2 +- kernel/sched/rt.c | 4 ++-- 5 files changed, 33 insertions(+), 4 deletions(-) -- 2.39.0
On 20/01/23 12:02, Wander Lairson Costa wrote:
> put_task_struct() decrements a usage counter and calls
> __put_task_struct() if the counter reaches zero.
>
> __put_task_struct() indirectly acquires a spinlock, which is a sleeping
> lock under PREEMPT_RT. Therefore, we can't call put_task_struct() in an
> atomic context in RT kernels.
>
> This patch series introduces put_task_struct_atomic_safe(), which defers
> the call to __put_task_struct() to a process context when compiled with
> PREEMPT_RT.
>
> It also fixes known problematic call sites.
>
Browsing around put_task_struct() callsites gives me the impression there
are more problematic call sites lurking around, which makes me wonder:
should we make the PREEMPT_RT put_task_struct() *always* be done via
call_rcu()?
The task's stack is actually always freed that way in put_task_stack(), cf.
e540bf3162e8 ("fork: Only cache the VMAP stack in finish_task_switch()")
> Changelog:
> ==========
>
> v2:
> * Add the put_task_struct_atomic_safe() function that is responsible for
> handling the conditions to call put_task_struct().
> * Replace put_task_struct() by put_task_struct_atomic_safe() in known
> atomic call sites.
>
> Wander Lairson Costa (4):
> sched/task: Add the put_task_struct_atomic_safe function
> sched/deadline: fix inactive_task_timer splat
> sched/rt: use put_task_struct_atomic_safe() to avoid potential splat
> sched/core: use put_task_struct_atomic_safe() to avoid potential splat
>
> include/linux/sched/task.h | 21 +++++++++++++++++++++
> kernel/fork.c | 8 ++++++++
> kernel/sched/core.c | 2 +-
> kernel/sched/deadline.c | 2 +-
> kernel/sched/rt.c | 4 ++--
> 5 files changed, 33 insertions(+), 4 deletions(-)
>
> --
> 2.39.0
On Fri, Jan 20, 2023 at 2:45 PM Valentin Schneider <vschneid@redhat.com> wrote:
>
> On 20/01/23 at 12:02, Wander Lairson Costa wrote:
> > put_task_struct() decrements a usage counter and calls
> > __put_task_struct() if the counter reaches zero.
> >
> > __put_task_struct() indirectly acquires a spinlock, which is a sleeping
> > lock under PREEMPT_RT. Therefore, we can't call put_task_struct() in an
> > atomic context in RT kernels.
> >
> > This patch series introduces put_task_struct_atomic_safe(), which defers
> > the call to __put_task_struct() to a process context when compiled with
> > PREEMPT_RT.
> >
> > It also fixes known problematic call sites.
> >
>
> Browsing around put_task_struct() callsites gives me the impression there
> are more problematic call sites lurking around, which makes me wonder:
> should we make the PREEMPT_RT put_task_struct() *always* be done via
> call_rcu()?
>
I thought about going on this route, but I was concerned about the
performance side effects this approach could bring. Another idea I had
was to check at runtime if we are in a preemptible context. Again,
this would have a (minor?) performance penalty.
> The task's stack is actually always freed that way in put_task_stack(), cf.
>
> e540bf3162e8 ("fork: Only cache the VMAP stack in finish_task_switch()")
>
> > Changelog:
> > ==========
> >
> > v2:
> > * Add the put_task_struct_atomic_safe() function that is responsible for
> > handling the conditions to call put_task_struct().
> > * Replace put_task_struct() by put_task_struct_atomic_safe() in known
> > atomic call sites.
> >
> > Wander Lairson Costa (4):
> > sched/task: Add the put_task_struct_atomic_safe function
> > sched/deadline: fix inactive_task_timer splat
> > sched/rt: use put_task_struct_atomic_safe() to avoid potential splat
> > sched/core: use put_task_struct_atomic_safe() to avoid potential splat
> >
> > include/linux/sched/task.h | 21 +++++++++++++++++++++
> > kernel/fork.c | 8 ++++++++
> > kernel/sched/core.c | 2 +-
> > kernel/sched/deadline.c | 2 +-
> > kernel/sched/rt.c | 4 ++--
> > 5 files changed, 33 insertions(+), 4 deletions(-)
> >
> > --
> > 2.39.0
>
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