Re: [PATCH v2 04/12] uprobes: revamp uprobe refcounting and lifetime management

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 04, 2024 at 10:03:48AM +0200, Peter Zijlstra wrote:

> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> index c98e3b3386ba..4aafb4485be7 100644
> --- a/kernel/trace/trace_uprobe.c
> +++ b/kernel/trace/trace_uprobe.c
> @@ -1112,7 +1112,8 @@ static void __probe_event_disable(struct trace_probe *tp)
>  		if (!tu->inode)
>  			continue;
>  
> -		uprobe_unregister(tu->inode, tu->offset, &tu->consumer);
> +		uprobe_unregister(tu->inode, tu->offset, &tu->consumer,
> +				  list_is_last(trace_probe_probe_list(tp), &tu->tp.list) ? 0 : URF_NO_SYNC);
>  		tu->inode = NULL;
>  	}
>  }


Hmm, that continue clause might ruin things. Still easy enough to add
uprobe_unregister_sync() and simpy always pass URF_NO_SYNC.

I really don't see why we should make this more complicated than it
needs to be.

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index 354cab634341..681741a51df3 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -115,7 +115,9 @@ extern int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm
 extern int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
 extern int uprobe_register_refctr(struct inode *inode, loff_t offset, loff_t ref_ctr_offset, struct uprobe_consumer *uc);
 extern int uprobe_apply(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, bool);
-extern void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc);
+#define URF_NO_SYNC	0x01
+extern void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, unsigned int flags);
+extern void uprobe_unregister_sync(void);
 extern int uprobe_mmap(struct vm_area_struct *vma);
 extern void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned long end);
 extern void uprobe_start_dup_mmap(void);
@@ -165,7 +167,7 @@ uprobe_apply(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, boo
 	return -ENOSYS;
 }
 static inline void
-uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
+uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, unsigned int flags)
 {
 }
 static inline int uprobe_mmap(struct vm_area_struct *vma)
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 0b7574a54093..d09f7b942076 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1145,7 +1145,7 @@ __uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
  * @offset: offset from the start of the file.
  * @uc: identify which probe if multiple probes are colocated.
  */
-void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
+void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc, unsigned int flags)
 {
 	scoped_guard (srcu, &uprobe_srcu) {
 		struct uprobe *uprobe = find_uprobe(inode, offset);
@@ -1157,10 +1157,17 @@ void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consume
 		mutex_unlock(&uprobe->register_mutex);
 	}
 
-	synchronize_srcu(&uprobe_srcu); // XXX amortize / batch
+	if (!(flags & URF_NO_SYNC))
+		synchronize_srcu(&uprobe_srcu);
 }
 EXPORT_SYMBOL_GPL(uprobe_unregister);
 
+void uprobe_unregister_sync(void)
+{
+	synchronize_srcu(&uprobe_srcu);
+}
+EXPORT_SYMBOL_GPL(uprobe_unregister_sync);
+
 /*
  * __uprobe_register - register a probe
  * @inode: the file in which the probe has to be placed.
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d1daeab1bbc1..1f6adabbb1e7 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3181,9 +3181,10 @@ static void bpf_uprobe_unregister(struct path *path, struct bpf_uprobe *uprobes,
 	u32 i;
 
 	for (i = 0; i < cnt; i++) {
-		uprobe_unregister(d_real_inode(path->dentry), uprobes[i].offset,
-				  &uprobes[i].consumer);
+		uprobe_unregister(d_real_inode(path->dentry), uprobes[i].offset, URF_NO_SYNC);
 	}
+	if (cnt > 0)
+		uprobe_unregister_sync();
 }
 
 static void bpf_uprobe_multi_link_release(struct bpf_link *link)
diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index c98e3b3386ba..6b64470a1c5c 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -1104,6 +1104,7 @@ static int trace_uprobe_enable(struct trace_uprobe *tu, filter_func_t filter)
 static void __probe_event_disable(struct trace_probe *tp)
 {
 	struct trace_uprobe *tu;
+	bool sync = false;
 
 	tu = container_of(tp, struct trace_uprobe, tp);
 	WARN_ON(!uprobe_filter_is_empty(tu->tp.event->filter));
@@ -1112,9 +1113,12 @@ static void __probe_event_disable(struct trace_probe *tp)
 		if (!tu->inode)
 			continue;
 
-		uprobe_unregister(tu->inode, tu->offset, &tu->consumer);
+		uprobe_unregister(tu->inode, tu->offset, &tu->consumer, URF_NO_SYNC);
+		sync = true;
 		tu->inode = NULL;
 	}
+	if (sync)
+		uprobe_unregister_sync();
 }
 
 static int probe_event_enable(struct trace_event_call *call,