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[104.155.91.135]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4376686130csm21600376f8f.1.2026.02.10.07.32.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Feb 2026 07:32:54 -0800 (PST) Date: Tue, 10 Feb 2026 15:32:50 +0000 From: Vincent Donnefort To: Steven Rostedt Cc: mhiramat@kernel.org, mathieu.desnoyers@efficios.com, linux-trace-kernel@vger.kernel.org, maz@kernel.org, oliver.upton@linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, jstultz@google.com, qperret@google.com, will@kernel.org, aneesh.kumar@kernel.org, kernel-team@android.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v11 07/30] tracing: Add non-consuming read to trace remotes Message-ID: References: <20260131132848.254084-1-vdonnefort@google.com> <20260131132848.254084-8-vdonnefort@google.com> <20260204185208.646a6d26@gandalf.local.home> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260204185208.646a6d26@gandalf.local.home> On Wed, Feb 04, 2026 at 06:52:08PM -0500, Steven Rostedt wrote: > On Sat, 31 Jan 2026 13:28:25 +0000 > Vincent Donnefort wrote: > > > -static struct trace_remote_iterator *trace_remote_iter(struct trace_remote *remote, int cpu) > > +static void __free_ring_buffer_iter(struct trace_remote_iterator *iter, int cpu) > > +{ > > + if (!iter->rb_iter) > > + return; > > Hmm, can't iter->rb_iter be NULL when iter->rb_iters[] is used? Arg yes, I missed that when I removed the union. And actually I don't think this can be called with iter->rb_iter or iter->rb_iter NULL anymore. > > > + > > + if (cpu != RING_BUFFER_ALL_CPUS) { > > + ring_buffer_read_finish(iter->rb_iter); > > + return; > > + } > > + > > + for_each_possible_cpu(cpu) { > > + if (iter->rb_iters[cpu]) > > + ring_buffer_read_finish(iter->rb_iters[cpu]); > > + } > > + > > + kfree(iter->rb_iters); > > +} > > + > > +static int __alloc_ring_buffer_iter(struct trace_remote_iterator *iter, int cpu) > > +{ > > + if (cpu != RING_BUFFER_ALL_CPUS) { > > + iter->rb_iter = ring_buffer_read_start(iter->remote->trace_buffer, cpu, GFP_KERNEL); > > + > > + return iter->rb_iter ? 0 : -ENOMEM; > > + } > > + > > + iter->rb_iters = kcalloc(nr_cpu_ids, sizeof(*iter->rb_iters), GFP_KERNEL); > > + if (!iter->rb_iters) > > + return -ENOMEM; > > + > > + for_each_possible_cpu(cpu) { > > + iter->rb_iters[cpu] = ring_buffer_read_start(iter->remote->trace_buffer, cpu, > > + GFP_KERNEL); > > + if (!iter->rb_iters[cpu]) { > > + __free_ring_buffer_iter(iter, RING_BUFFER_ALL_CPUS); > > For instance, we call __free_ring_buffer_iter() here, but I don't see > iter->rb_iter being set. > > -- Steve > > > > + return -ENOMEM; > > + } > > + } > > + > > + return 0; > > +} > > + [...] > > +static void *trace_next(struct seq_file *m, void *v, loff_t *pos) > > +{ > > + struct trace_remote_iterator *iter = m->private; > > + > > + ++*pos; > > + > > + if (!iter || !trace_remote_iter_read_event(iter)) > > + return NULL; > > + > > + trace_remote_iter_move(iter); > > + iter->pos++; > > + > > + return iter; > > +} > > + > > +static void *trace_start(struct seq_file *m, loff_t *pos) > > +{ > > + struct trace_remote_iterator *iter = m->private; > > + loff_t i; > > + > > FYI, this is where you take locks for iteration of files. > > > + if (!iter) > > + return NULL; > > + > > + if (!*pos) { > > + iter->pos = -1; > > + return trace_next(m, NULL, &i); > > + } > > + > > + i = iter->pos; > > + while (i < *pos) { > > + iter = trace_next(m, NULL, &i); > > + if (!iter) > > + return NULL; > > + } > > + > > + return iter; > > +} > > + > > +static int trace_show(struct seq_file *m, void *v) > > +{ > > + struct trace_remote_iterator *iter = v; > > + > > + trace_seq_init(&iter->seq); > > + > > + if (trace_remote_iter_print_event(iter)) { > > + seq_printf(m, "[EVENT %d PRINT TOO BIG]\n", iter->evt->id); > > + return 0; > > + } > > + > > + return trace_print_seq(m, &iter->seq); > > +} > > + > > +static void trace_stop(struct seq_file *s, void *v) { } > > And stop is where you release the locks. > > > + > > +static const struct seq_operations trace_sops = { > > + .start = trace_start, > > + .next = trace_next, > > + .show = trace_show, > > + .stop = trace_stop, > > +}; > > + > > +static int trace_open(struct inode *inode, struct file *filp) > > +{ > > + struct trace_remote *remote = inode->i_private; > > + struct trace_remote_iterator *iter = NULL; > > + int cpu = tracing_get_cpu(inode); > > + int ret; > > + > > + if (!(filp->f_mode & FMODE_READ)) > > + return 0; > > + > > + guard(mutex)(&remote->lock); > > + > > + iter = trace_remote_iter(remote, cpu, TRI_NONCONSUMING); > > + if (IS_ERR(iter)) > > + return PTR_ERR(iter); > > So if iter is bad we exit out here. > > > + > > + ret = seq_open(filp, &trace_sops); > > + if (ret) { > > + trace_remote_iter_free(iter); > > + return ret; > > + } > > + > > + if (iter) > > Why test if iter exists here? We only test IS_ERR. iter will be NULL if the buffer isn't loaded and the userspace output would be empty. But anyway if I move the locking into start/stop this line will go away! > > > + trace_remote_iter_read_start(iter); > > But still, the above grabs locks in the open, where it can return to user > space while still holding the locks? That's a no-no. > > You can use the seq file start and stop for locking. > > -- Steve > [...]