From f85cd289bcfca774e34794cd751a68af711f25fa Mon Sep 17 00:00:00 2001
From: Avi Kivity <avi@scylladb.com>
Date: Sun, 16 Jan 2022 18:22:29 +0200
Subject: [PATCH] Merge "repair: make sure there is one permit per repair with
 count res" from Botond

"
Repair obtains a permit for each repair-meta instance it creates. This
permit is supposed to track all resources consumed by that repair as
well as ensure concurrency limit is respected. However when the
non-local reader path is used (shard config of master != shard config of
follower), a second permit will be obtained -- for the shard reader of
the multishard reader. This creates a situation where the repair-meta's
permit can block the shard permit, creating a deadlock situation.
This patch solves this by dropping the count resource on the
repair-meta's permit when a non-local reader path is executed -- that is
a multishard reader is created.

Fixes: #9751
"

* 'repair-double-permit-block/v4' of https://github.com/denesb/scylla:
  repair: make sure there is one permit per repair with count res
  reader_permit: add release_base_resource()

(cherry picked from commit 52b7778ae69190adbaccb15364de22b1045b6399)
---
 reader_concurrency_semaphore.cc | 8 ++++++++
 reader_permit.hh                | 2 ++
 repair/row_level.cc             | 4 ++++
 3 files changed, 14 insertions(+)

diff --git a/reader_concurrency_semaphore.cc b/reader_concurrency_semaphore.cc
index 9fb18317c4..006139a265 100644
--- a/reader_concurrency_semaphore.cc
+++ b/reader_concurrency_semaphore.cc
@@ -249,6 +249,10 @@ public:
         return _base_resources;
     }
 
+    void release_base_resources() noexcept {
+        _semaphore.signal(std::exchange(_base_resources, {}));
+    }
+
     sstring description() const {
         return format("{}.{}:{}",
                 _schema ? _schema->ks_name() : "*",
@@ -394,6 +398,10 @@ reader_resources reader_permit::base_resources() const {
     return _impl->base_resources();
 }
 
+void reader_permit::release_base_resources() noexcept {
+    return _impl->release_base_resources();
+}
+
 sstring reader_permit::description() const {
     return _impl->description();
 }
diff --git a/reader_permit.hh b/reader_permit.hh
index 8a0c85e659..6cefbda790 100644
--- a/reader_permit.hh
+++ b/reader_permit.hh
@@ -161,6 +161,8 @@ public:
 
     reader_resources base_resources() const;
 
+    void release_base_resources() noexcept;
+
     sstring description() const;
 
     db::timeout_clock::time_point timeout() const noexcept;
diff --git a/repair/row_level.cc b/repair/row_level.cc
index 4a4e1af66b..02217ad5dc 100644
--- a/repair/row_level.cc
+++ b/repair/row_level.cc
@@ -407,6 +407,10 @@ public:
                     {},
                     mutation_reader::forwarding::no);
         } else {
+            // We can't have two permits with count resource for 1 repair.
+            // So we release the one on _permit so the only one is the one the
+            // shard reader will obtain.
+            _permit.release_base_resources();
             _reader = make_multishard_streaming_reader(db, _schema, _permit, [this] {
                 auto shard_range = _sharder.next();
                 if (shard_range) {