Re: [PATCH perf/core] uprobes: remove too strict lockdep_assert() condition in hprobe_expire()
From: Jiri Olsa <hidden>
Date: 2025-02-26 10:41:17
Also in:
bpf, lkml
On Tue, Feb 25, 2025 at 02:32:14PM -0800, Andrii Nakryiko wrote:
hprobe_expire() is used to atomically switch pending uretprobe instance
(struct return_instance) from being SRCU protected to be refcounted.
This can be done from background timer thread, or synchronously within
current thread when task is forked.
In the former case, return_instance has to be protected through RCU read
lock, and that's what hprobe_expire() used to check with
lockdep_assert(rcu_read_lock_held()).
But in the latter case (hprobe_expire() called from dup_utask()) there
is no RCU lock being held, and it's both unnecessary and incovenient.
Inconvenient due to the intervening memory allocations inside
dup_return_instance()'s loop. Unnecessary because dup_utask() is called
synchronously in current thread, and no uretprobe can run at that point,
so return_instance can't be freed either.
So drop rcu_read_lock_held() condition, and expand corresponding comment
to explain necessary lifetime guarantees. lockdep_assert()-detected
issue is a false positive.
Fixes: dd1a7567784e ("uprobes: SRCU-protect uretprobe lifetime (with timeout)")
Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>lgtm Reviewed-by: Jiri Olsa <jolsa@kernel.org> jirka
quoted hunk ↗ jump to hunk
--- kernel/events/uprobes.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-)diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c index e783da1d1762..4d2140cab7ec 100644 --- a/kernel/events/uprobes.c +++ b/kernel/events/uprobes.c@@ -762,10 +762,14 @@ static struct uprobe *hprobe_expire(struct hprobe *hprobe, bool get) enum hprobe_state hstate; /* - * return_instance's hprobe is protected by RCU. - * Underlying uprobe is itself protected from reuse by SRCU. + * Caller should guarantee that return_instance is not going to be + * freed from under us. This can be achieved either through holding + * rcu_read_lock() or by owning return_instance in the first place. + * + * Underlying uprobe is itself protected from reuse by SRCU, so ensure + * SRCU lock is held properly. */ - lockdep_assert(rcu_read_lock_held() && srcu_read_lock_held(&uretprobes_srcu)); + lockdep_assert(srcu_read_lock_held(&uretprobes_srcu)); hstate = READ_ONCE(hprobe->state); switch (hstate) {-- 2.43.5