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[37.188.174.24]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-58cf5bdeda0sm4614386a12.73.2024.07.06.10.00.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 06 Jul 2024 10:00:38 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sat, 6 Jul 2024 19:00:34 +0200 To: Oleg Nesterov Cc: Andrii Nakryiko , linux-trace-kernel@vger.kernel.org, rostedt@goodmis.org, mhiramat@kernel.org, peterz@infradead.org, mingo@redhat.com, bpf@vger.kernel.org, paulmck@kernel.org, clm@meta.com Subject: Re: [PATCH v2 04/12] uprobes: revamp uprobe refcounting and lifetime management Message-ID: References: <20240701223935.3783951-1-andrii@kernel.org> <20240701223935.3783951-5-andrii@kernel.org> <20240705153705.GA18551@redhat.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240705153705.GA18551@redhat.com> On Fri, Jul 05, 2024 at 05:37:05PM +0200, Oleg Nesterov wrote: > Tried to read this patch, but I fail to understand it. It looks > obvioulsy wrong to me, see below. > > I tend to agree with the comments from Peter, but lets ignore them > for the moment. > > On 07/01, Andrii Nakryiko wrote: > > > > static void put_uprobe(struct uprobe *uprobe) > > { > > - if (refcount_dec_and_test(&uprobe->ref)) { > > + s64 v; > > + > > + /* > > + * here uprobe instance is guaranteed to be alive, so we use Tasks > > + * Trace RCU to guarantee that uprobe won't be freed from under us, if > > + * we end up being a losing "destructor" inside uprobe_treelock'ed > > + * section double-checking uprobe->ref value below. > > + * Note call_rcu_tasks_trace() + uprobe_free_rcu below. > > + */ > > + rcu_read_lock_trace(); > > + > > + v = atomic64_add_return(UPROBE_REFCNT_PUT, &uprobe->ref); > > + > > + if (unlikely((u32)v == 0)) { > > I must have missed something, but how can this ever happen? > > Suppose uprobe_register(inode) is called the 1st time. To simplify, suppose > that this binary is not used, so _register() doesn't install breakpoints/etc. > > IIUC, with this change (u32)uprobe->ref == 1 when uprobe_register() succeeds. > > Now suppose that uprobe_unregister() is called right after that. It does > > uprobe = find_uprobe(inode, offset); > > this increments the counter, (u32)uprobe->ref == 2 > > __uprobe_unregister(...); > > this wont't change the counter, __uprobe_unregister calls delete_uprobe that calls put_uprobe ? jirka > > put_uprobe(uprobe); > > this drops the reference added by find_uprobe(), (u32)uprobe->ref == 1. > > Where should the "final" put_uprobe() come from? > > IIUC, this patch lacks another put_uprobe() after consumer_del(), no? > > Oleg. >