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

On Fri Jul 10, 2026 at 11:23 PM CEST, Andrii Nakryiko wrote:

On Mon, Jul 6, 2026 at 10:28 AM Puranjay Mohan [off-list ref] wrote:

quoted

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.

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