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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

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.

> [...]