From 10aba174c999c795eb3b2170cfca5597530a7eab Mon Sep 17 00:00:00 2001
From: Joonas Koivunen <joonas@neon.tech>
Date: Tue, 4 Jul 2023 17:40:51 +0300
Subject: [PATCH] metrics: Remove comments regarding upgradeable rwlocks
 (#4622)

Closes #4001 by removing the comments alluding towards
upgradeable/downgradeable RwLocks.
---
 pageserver/src/metrics.rs      | 5 -----
 pageserver/src/virtual_file.rs | 9 ---------
 2 files changed, 14 deletions(-)

diff --git a/pageserver/src/metrics.rs b/pageserver/src/metrics.rs
index 8765aa6aff..8f85488aec 100644
--- a/pageserver/src/metrics.rs
+++ b/pageserver/src/metrics.rs
@@ -968,7 +968,6 @@ impl RemoteTimelineClientMetrics {
         op_kind: &RemoteOpKind,
         status: &'static str,
     ) -> Histogram {
-        // XXX would be nice to have an upgradable RwLock
         let mut guard = self.remote_operation_time.lock().unwrap();
         let key = (file_kind.as_str(), op_kind.as_str(), status);
         let metric = guard.entry(key).or_insert_with(move || {
@@ -990,7 +989,6 @@ impl RemoteTimelineClientMetrics {
         file_kind: &RemoteOpFileKind,
         op_kind: &RemoteOpKind,
     ) -> IntGauge {
-        // XXX would be nice to have an upgradable RwLock
         let mut guard = self.calls_unfinished_gauge.lock().unwrap();
         let key = (file_kind.as_str(), op_kind.as_str());
         let metric = guard.entry(key).or_insert_with(move || {
@@ -1011,7 +1009,6 @@ impl RemoteTimelineClientMetrics {
         file_kind: &RemoteOpFileKind,
         op_kind: &RemoteOpKind,
     ) -> Histogram {
-        // XXX would be nice to have an upgradable RwLock
         let mut guard = self.calls_started_hist.lock().unwrap();
         let key = (file_kind.as_str(), op_kind.as_str());
         let metric = guard.entry(key).or_insert_with(move || {
@@ -1032,7 +1029,6 @@ impl RemoteTimelineClientMetrics {
         file_kind: &RemoteOpFileKind,
         op_kind: &RemoteOpKind,
     ) -> IntCounter {
-        // XXX would be nice to have an upgradable RwLock
         let mut guard = self.bytes_started_counter.lock().unwrap();
         let key = (file_kind.as_str(), op_kind.as_str());
         let metric = guard.entry(key).or_insert_with(move || {
@@ -1053,7 +1049,6 @@ impl RemoteTimelineClientMetrics {
         file_kind: &RemoteOpFileKind,
         op_kind: &RemoteOpKind,
     ) -> IntCounter {
-        // XXX would be nice to have an upgradable RwLock
         let mut guard = self.bytes_finished_counter.lock().unwrap();
         let key = (file_kind.as_str(), op_kind.as_str());
         let metric = guard.entry(key).or_insert_with(move || {
diff --git a/pageserver/src/virtual_file.rs b/pageserver/src/virtual_file.rs
index fb216123c1..b86d14f158 100644
--- a/pageserver/src/virtual_file.rs
+++ b/pageserver/src/virtual_file.rs
@@ -302,15 +302,6 @@ impl VirtualFile {
             .observe_closure_duration(|| self.open_options.open(&self.path))?;
 
         // Perform the requested operation on it
-        //
-        // TODO: We could downgrade the locks to read mode before calling
-        // 'func', to allow a little bit more concurrency, but the standard
-        // library RwLock doesn't allow downgrading without releasing the lock,
-        // and that doesn't seem worth the trouble.
-        //
-        // XXX: `parking_lot::RwLock` can enable such downgrades, yet its implementation is fair and
-        // may deadlock on subsequent read calls.
-        // Simply replacing all `RwLock` in project causes deadlocks, so use it sparingly.
         let result = STORAGE_IO_TIME
             .with_label_values(&[op, &self.tenant_id, &self.timeline_id])
             .observe_closure_duration(|| func(&file));