[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20240708180837.GC27299@noisy.programming.kicks-ass.net>
Date: Mon, 8 Jul 2024 20:08:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: mingo@...nel.org, andrii@...nel.org, linux-kernel@...r.kernel.org,
rostedt@...dmis.org, mhiramat@...nel.org, jolsa@...nel.org,
clm@...a.com, paulmck@...nel.org
Subject: Re: [PATCH 04/10] perf/uprobe: RCU-ify find_uprobe()
On Mon, Jul 08, 2024 at 06:35:45PM +0200, Oleg Nesterov wrote:
> I hate to say this again, but I'll try to read this series later ;)
>
> But let me ask...
:-)
> On 07/08, Peter Zijlstra wrote:
> >
> > +static void uprobe_free_rcu(struct rcu_head *rcu)
> > +{
> > + struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
> > + kfree(uprobe);
> > +}
> > +
> > static void put_uprobe(struct uprobe *uprobe)
> > {
> > if (refcount_dec_and_test(&uprobe->ref)) {
> > @@ -604,7 +612,7 @@ static void put_uprobe(struct uprobe *up
> > mutex_lock(&delayed_uprobe_lock);
> > delayed_uprobe_remove(uprobe, NULL);
> > mutex_unlock(&delayed_uprobe_lock);
> > - kfree(uprobe);
> > + call_rcu(&uprobe->rcu, uprobe_free_rcu);
>
> kfree_rcu() ?
I can never remember how that works, also this will very soon be
call_srcu().
> > static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
> > {
> > - struct uprobe *uprobe;
> > + unsigned int seq;
> >
> > - read_lock(&uprobes_treelock);
> > - uprobe = __find_uprobe(inode, offset);
> > - read_unlock(&uprobes_treelock);
> > + guard(rcu)();
> >
> > - return uprobe;
> > + do {
> > + seq = read_seqcount_begin(&uprobes_seqcount);
> > + struct uprobe *uprobe = __find_uprobe(inode, offset);
> > + if (uprobe) {
> > + /*
> > + * Lockless RB-tree lookups are prone to false-negatives.
> > + * If they find something, it's good.
>
> Is it true in this case?
>
> Suppose we have uprobe U which has no extra refs, so uprobe_unregister()
> called by the task X should remove it from uprobes_tree and kfree.
>
> Suppose that the task T hits the breakpoint and enters handle_swbp().
>
> Now,
>
> - X calls find_uprobe(), this increments U->ref from 1 to 2
>
> register_for_each_vma() succeeds
>
> X enters delete_uprobe()
>
> - T calls find_active_uprobe() -> find_uprobe()
>
> __read_seqcount_begin__read_seqcount_begin() returns an even number
>
> __find_uprobe() -> rb_find_rcu() succeeds
>
> - X continues and returns from delete_uprobe(), U->ref == 1
>
> then it does the final uprobe_unregister()->put_uprobe(U),
> refcount_dec_and_test() succeeds, X calls call_rcu(uprobe_free_rcu).
>
> - T does get_uprobe() which changes U->ref from 0 to 1, __find_uprobe()
> returns, find_uprobe() doesn't check read_seqcount_retry().
I think you're right. However, this get_uprobe() will go away in a few
patches.
But yeah, I should perhaps have been more careful when splitting things
up :/
Powered by blists - more mailing lists