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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 30, 2026 at 03:27:27PM -0700, Andrii Nakryiko wrote:
> On Tue, Jul 21, 2026 at 12:44 PM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> 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@kernel.org> 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@kernel.org>
> > >> ---
> > >>  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@kernel.org>
> > >
> >
> > Hi Peter, Ingo,
> >
> > Gentle ping for this.
> >
> 
> Another ping, please consider applying, thanks!

Got them, thanks!