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

Alexei Starovoitov
2024-09-10T02:52:06Z

On Mon, Sep 9, 2024 at 3:49 PM Andrii Nakryiko <andrii@kernel.org> 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@kernel.org>
> ---
> 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.