Re: [PATCH 1/3] uprobes: allow put_uprobe() from non-sleepable softirq context

[Andrii Nakryiko]

On Mon, Sep 9, 2024 at 7:52 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, Sep 9, 2024 at 3:49 PM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> >
> > Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which
> > makes it unsuitable to be called from more restricted context like softirq.
> >
> > Let's make put_uprobe() agnostic to the context in which it is called,
> > and use work queue to defer the mutex-protected clean up steps.
> >
> > To avoid unnecessarily increasing the size of struct uprobe, we colocate
> > work_struct in parallel with rb_node and rcu, both of which are unused
> > by the time we get to schedule clean up work.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> >  kernel/events/uprobes.c | 30 +++++++++++++++++++++++++++---
> >  1 file changed, 27 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index a2e6a57f79f2..377bd524bc8b 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -27,6 +27,7 @@
> >  #include <linux/shmem_fs.h>
> >  #include <linux/khugepaged.h>
> >  #include <linux/rcupdate_trace.h>
> > +#include <linux/workqueue.h>
> >
> >  #include <linux/uprobes.h>
> >
> > @@ -54,14 +55,20 @@ DEFINE_STATIC_PERCPU_RWSEM(dup_mmap_sem);
> >  #define UPROBE_COPY_INSN       0
> >
> >  struct uprobe {
> > -       struct rb_node          rb_node;        /* node in the rb tree */
> > +       union {
> > +               struct {
> > +                       struct rb_node          rb_node;        /* node in the rb tree */
> > +                       struct rcu_head         rcu;
> > +               };
> > +               /* work is used only during freeing, rcu and rb_node are unused at that point */
> > +               struct work_struct work;
> > +       };
> >         refcount_t              ref;
> >         struct rw_semaphore     register_rwsem;
> >         struct rw_semaphore     consumer_rwsem;
> >         struct list_head        pending_list;
> >         struct list_head        consumers;
> >         struct inode            *inode;         /* Also hold a ref to inode */
> > -       struct rcu_head         rcu;
> >         loff_t                  offset;
> >         loff_t                  ref_ctr_offset;
> >         unsigned long           flags;
> > @@ -620,11 +627,28 @@ static inline bool uprobe_is_active(struct uprobe *uprobe)
> >         return !RB_EMPTY_NODE(&uprobe->rb_node);
> >  }
> >
> > +static void uprobe_free_deferred(struct work_struct *work)
> > +{
> > +       struct uprobe *uprobe = container_of(work, struct uprobe, work);
> > +
> > +       /*
> > +        * If application munmap(exec_vma) before uprobe_unregister()
> > +        * gets called, we don't get a chance to remove uprobe from
> > +        * delayed_uprobe_list from remove_breakpoint(). Do it here.
> > +        */
> > +       mutex_lock(&delayed_uprobe_lock);
> > +       delayed_uprobe_remove(uprobe, NULL);
> > +       mutex_unlock(&delayed_uprobe_lock);
> > +
> > +       kfree(uprobe);
> > +}
> > +
> >  static void uprobe_free_rcu(struct rcu_head *rcu)
> >  {
> >         struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
> >
> > -       kfree(uprobe);
> > +       INIT_WORK(&uprobe->work, uprobe_free_deferred);
> > +       schedule_work(&uprobe->work);
> >  }
> >
> >  static void put_uprobe(struct uprobe *uprobe)
>
> It seems put_uprobe hunk was lost, since the patch is not doing
> what commit log describes.


Hmm, put_uprobe() has:

call_srcu(&uprobes_srcu, &uprobe->rcu, uprobe_free_rcu);

at the end (see [0], which added that), so we do schedule_work() in
RCU callback, similarly to what we do with bpf_map freeing in the BPF
subsystem.

This patch set is based on the latest tip/perf/core (and also assuming
the RCU Tasks Trace patch that mysteriously disappeared is actually
there, hopefully it will just as magically be restored).

  [0] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=perf/core&id=8617408f7a01e94ce1f73e40a7704530e5dfb25c