Re: [PATCHv5 bpf-next 04/28] bpf: Use mutex lock pool for bpf trampolines

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Apr 17, 2026 at 08:10:24PM +0000, bot+bpf-ci@kernel.org wrote:
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> 
> [ ... ]
> 
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -30,6 +30,34 @@ #define TRAMPOLINE_TABLE_SIZE (1 << TRAMPOLINE_HASH_BITS)
> >  /* serializes access to trampoline tables */
> >  static DEFINE_MUTEX(trampoline_mutex);
> >
> > +/*
> > + * We keep 32 trampoline locks (5 bits) in the pool, because there is
> > + * 48 (MAX_LOCK_DEPTH) locks limit allowed to be simultaneously held
> > + * by task. Each lock has its own lockdep key to keep it simple.
> > + */
> 
> The comment explains the 32 count (MAX_LOCK_DEPTH limit), but should it
> also explain why each lock has its own lock_class_key?
> 
> Without that explanation, it is not obvious that distinct keys are required
> to avoid lockdep "recursive locking" warnings when trampoline_lock_all()
> acquires all 32 pool mutexes simultaneously.
> 
> This was raised by bot+bpf-ci@kernel.org in v3 review:
> https://lore.kernel.org/bpf/31ae46274a3157f2b9840e1a09b2698d1ec0cfd461737ff460c2d3349a9f0054@mail.kernel.org/
> 
> The author acknowledged it with "will add", but the current comment only
> says "to keep it simple" without addressing the recursive locking concern.

will add more details

jirka

> 
> > +#define TRAMPOLINE_LOCKS_BITS 5
> > +#define TRAMPOLINE_LOCKS_TABLE_SIZE (1 << TRAMPOLINE_LOCKS_BITS)
> > +
> > +static struct {
> > +	struct mutex mutex;
> > +	struct lock_class_key key;
> > +} trampoline_locks[TRAMPOLINE_LOCKS_TABLE_SIZE];
> 
> [ ... ]
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/24583317711