Re: [PATCH bpf-next 2/2] uprobes: Switch uretprobes_srcu to SRCU-fast-updown

[Andrii Nakryiko]

On Tue, Jul 21, 2026 at 12:44 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> On Fri Jul 10, 2026 at 11:23 PM CEST, Andrii Nakryiko wrote:
> > On Mon, Jul 6, 2026 at 10:28 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
> >>
> >> uretprobes_srcu currently uses normal SRCU, which issues
> >> two smp_mb() per read lock/unlock pair. This overhead is
> >> paid on every uretprobe hit.
> >>
> >> Switch to SRCU-fast-updown, which eliminates the per-reader
> >> memory barriers by moving the ordering cost to the
> >> grace-period side (synchronize_rcu() instead of smp_mb()).
> >> This is acceptable because grace periods (uprobe
> >> unregistration) are infrequent compared to reader-side
> >> uretprobe hits.
> >>
> >> The updown flavor is required because the SRCU read lock is
> >> taken in prepare_uretprobe() when a return instance is
> >> created and is held until that return instance is finalized.
> >> The traced thread returns to user space in between, so the
> >> lock is inherently released in a different context from
> >> where it was acquired: on the normal return path via
> >> uprobe_handle_trampoline() -> hprobe_finalize(), or from
> >> ri_timer() (expiry) or dup_utask() (fork) via
> >> hprobe_expire(). srcu_down_read_fast() / srcu_up_read_fast()
> >> are designed for this acquire-here / release-elsewhere
> >> pattern and, unlike the same-context srcu_read_lock_fast()
> >> variant, do not carry the lockdep read-side tracking that
> >> would warn on it.
> >>
> >> The short, same-context SRCU sections in ri_timer() and
> >> dup_utask() (which guard the uprobe against reuse across the
> >> hprobe_expire() cmpxchg) instead use guard(srcu_fast_updown)
> >> for proper lockdep coverage.
> >>
> >> Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
> >> ---
> >>  include/linux/uprobes.h |  5 +++--
> >>  kernel/events/uprobes.c | 29 +++++++++++++++++------------
> >>  2 files changed, 20 insertions(+), 14 deletions(-)
> >>
> >
> > Very straightforward replacement and will improve performance, thanks!
> > LGTM. Peter or Mingo, I assume you guys will want to route this
> > through your tip tree, right?
> >
> > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> >
>
> Hi Peter, Ingo,
>
> Gentle ping for this.
>

Another ping, please consider applying, thanks!

> > [...]
>