Re: [PATCH v2 tip/perf/core 1/2] uprobes: allow put_uprobe() from non-sleepable softirq context
From: Andrii Nakryiko <hidden>
Date: 2024-10-18 18:22:15
Also in:
bpf, lkml
On Fri, Oct 18, 2024 at 1:26 AM Peter Zijlstra [off-list ref] wrote:
On Mon, Oct 07, 2024 at 05:25:55PM -0700, Andrii Nakryiko wrote:quoted
Currently put_uprobe() might trigger mutex_lock()/mutex_unlock(), which makes it unsuitable to be called from more restricted context like softirq.This is delayed_uprobe_lock, right?
Not just delated_uprobe_lock, there is also uprobes_treelock (I forgot to update the commit message to mention that). Oleg had concerns (see [0]) with that being taken from the timer thread, so I just moved all of the locking into deferred work callback. [0] https://lore.kernel.org/linux-trace-kernel/20240915144910.GA27726@redhat.com/ (local)
So can't we do something like so instead?
I'll need to look at this more thoroughly (and hopefully Oleg will get a chance as well), dropping lock from delayed_ref_ctr_inc() is a bit scary, but might be ok. But generally speaking, what's your concern with doing deferred work in put_uprobe()? It's not a hot path by any means, worst case we'll have maybe thousands of uprobes attached/detached.
quoted hunk ↗ jump to hunk
--- kernel/events/uprobes.c | 40 +++++++++++++++++++++++----------------- 1 file changed, 23 insertions(+), 17 deletions(-)diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c index 2a0059464383..d17a9046de35 100644 --- a/kernel/events/uprobes.c +++ b/kernel/events/uprobes.c@@ -83,9 +83,11 @@ struct delayed_uprobe { struct list_head list; struct uprobe *uprobe; struct mm_struct *mm; + struct rcu_head rcu; }; -static DEFINE_MUTEX(delayed_uprobe_lock); +/* XXX global state; use per mm list instead ? */ +static DEFINE_SPINLOCK(delayed_uprobe_lock); static LIST_HEAD(delayed_uprobe_list); /*@@ -289,9 +291,11 @@ delayed_uprobe_check(struct uprobe *uprobe, struct mm_struct *mm) { struct delayed_uprobe *du; - list_for_each_entry(du, &delayed_uprobe_list, list) + guard(rcu)(); + list_for_each_entry_rcu(du, &delayed_uprobe_list, list) { if (du->uprobe == uprobe && du->mm == mm) return du; + } return NULL; }@@ -308,7 +312,8 @@ static int delayed_uprobe_add(struct uprobe *uprobe, struct mm_struct *mm) du->uprobe = uprobe; du->mm = mm; - list_add(&du->list, &delayed_uprobe_list); + scoped_guard(spinlock, &delayed_uprobe_lock) + list_add_rcu(&du->list, &delayed_uprobe_list); return 0; }@@ -316,19 +321,21 @@ static void delayed_uprobe_delete(struct delayed_uprobe *du) { if (WARN_ON(!du)) return; - list_del(&du->list); - kfree(du); + scoped_guard(spinlock, &delayed_uprobe_lock) + list_del(&du->list); + kfree_rcu(du, rcu); } static void delayed_uprobe_remove(struct uprobe *uprobe, struct mm_struct *mm) { - struct list_head *pos, *q; struct delayed_uprobe *du; + struct list_head *pos; if (!uprobe && !mm) return; - list_for_each_safe(pos, q, &delayed_uprobe_list) { + guard(rcu)(); + list_for_each_rcu(pos, &delayed_uprobe_list) { du = list_entry(pos, struct delayed_uprobe, list); if (uprobe && du->uprobe != uprobe)@@ -434,12 +441,10 @@ static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm, return ret; } - mutex_lock(&delayed_uprobe_lock); if (d > 0) ret = delayed_uprobe_add(uprobe, mm); else delayed_uprobe_remove(uprobe, mm); - mutex_unlock(&delayed_uprobe_lock); return ret; }@@ -645,9 +650,7 @@ static void put_uprobe(struct uprobe *uprobe) * gets called, we don't get a chance to remove uprobe from * delayed_uprobe_list from remove_breakpoint(). Do it here. */ - mutex_lock(&delayed_uprobe_lock); delayed_uprobe_remove(uprobe, NULL); - mutex_unlock(&delayed_uprobe_lock); call_rcu_tasks_trace(&uprobe->rcu, uprobe_free_rcu); }@@ -1350,13 +1353,18 @@ static void build_probe_list(struct inode *inode, /* @vma contains reference counter, not the probed instruction. */ static int delayed_ref_ctr_inc(struct vm_area_struct *vma) { - struct list_head *pos, *q; struct delayed_uprobe *du; + struct list_head *pos; unsigned long vaddr; int ret = 0, err = 0; - mutex_lock(&delayed_uprobe_lock); - list_for_each_safe(pos, q, &delayed_uprobe_list) { + /* + * delayed_uprobe_list is added to when the ref_ctr is not mapped + * and is consulted (this function) when adding maps. And since + * mmap_lock serializes these, it is not possible miss an entry. + */ + guard(rcu)(); + list_for_each_rcu(pos, &delayed_uprobe_list) { du = list_entry(pos, struct delayed_uprobe, list); if (du->mm != vma->vm_mm ||@@ -1370,9 +1378,9 @@ static int delayed_ref_ctr_inc(struct vm_area_struct *vma) if (!err) err = ret; } + delayed_uprobe_delete(du); } - mutex_unlock(&delayed_uprobe_lock); return err; }@@ -1596,9 +1604,7 @@ void uprobe_clear_state(struct mm_struct *mm) { struct xol_area *area = mm->uprobes_state.xol_area; - mutex_lock(&delayed_uprobe_lock); delayed_uprobe_remove(NULL, mm); - mutex_unlock(&delayed_uprobe_lock); if (!area) return;