From e0d9472ced9a0bfbdc41623340c0e0937bda6825 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Thu, 3 Feb 2022 01:39:49 +0100
Subject: [PATCH] Kernel: Protect Inode's list of watchers with spinlock
 instead of mutex

---
 Kernel/FileSystem/Inode.cpp | 51 +++++++++++++++++++------------------
 Kernel/FileSystem/Inode.h   |  4 +--
 2 files changed, 28 insertions(+), 27 deletions(-)

diff --git a/Kernel/FileSystem/Inode.cpp b/Kernel/FileSystem/Inode.cpp
index 69945ba8002..078518b8114 100644
--- a/Kernel/FileSystem/Inode.cpp
+++ b/Kernel/FileSystem/Inode.cpp
@@ -93,9 +93,9 @@ Inode::Inode(FileSystem& fs, InodeIndex index)
 
 Inode::~Inode()
 {
-    for (auto& watcher : m_watchers) {
+    m_watchers.for_each([&](auto& watcher) {
         watcher->unregister_by_inode({}, identifier());
-    }
+    });
 }
 
 void Inode::will_be_destroyed()
@@ -156,17 +156,19 @@ bool Inode::unbind_socket()
 
 ErrorOr<void> Inode::register_watcher(Badge<InodeWatcher>, InodeWatcher& watcher)
 {
-    MutexLocker locker(m_inode_lock);
-    VERIFY(!m_watchers.contains(&watcher));
-    TRY(m_watchers.try_set(&watcher));
-    return {};
+    return m_watchers.with([&](auto& watchers) -> ErrorOr<void> {
+        VERIFY(!watchers.contains(&watcher));
+        TRY(watchers.try_set(&watcher));
+        return {};
+    });
 }
 
 void Inode::unregister_watcher(Badge<InodeWatcher>, InodeWatcher& watcher)
 {
-    MutexLocker locker(m_inode_lock);
-    VERIFY(m_watchers.contains(&watcher));
-    m_watchers.remove(&watcher);
+    m_watchers.with([&](auto& watchers) {
+        VERIFY(watchers.contains(&watcher));
+        watchers.remove(&watcher);
+    });
 }
 
 ErrorOr<NonnullRefPtr<FIFO>> Inode::fifo()
@@ -197,49 +199,43 @@ void Inode::set_metadata_dirty(bool metadata_dirty)
     if (m_metadata_dirty) {
         // FIXME: Maybe we should hook into modification events somewhere else, I'm not sure where.
         //        We don't always end up on this particular code path, for instance when writing to an ext2fs file.
-        for (auto& watcher : m_watchers) {
+        m_watchers.for_each([&](auto& watcher) {
             watcher->notify_inode_event({}, identifier(), InodeWatcherEvent::Type::MetadataModified);
-        }
+        });
     }
 }
 
 void Inode::did_add_child(InodeIdentifier, StringView name)
 {
-    MutexLocker locker(m_inode_lock);
-
-    for (auto& watcher : m_watchers) {
+    m_watchers.for_each([&](auto& watcher) {
         watcher->notify_inode_event({}, identifier(), InodeWatcherEvent::Type::ChildCreated, name);
-    }
+    });
 }
 
 void Inode::did_remove_child(InodeIdentifier, StringView name)
 {
-    MutexLocker locker(m_inode_lock);
-
     if (name == "." || name == "..") {
         // These are just aliases and are not interesting to userspace.
         return;
     }
 
-    for (auto& watcher : m_watchers) {
+    m_watchers.for_each([&](auto& watcher) {
         watcher->notify_inode_event({}, identifier(), InodeWatcherEvent::Type::ChildDeleted, name);
-    }
+    });
 }
 
 void Inode::did_modify_contents()
 {
-    MutexLocker locker(m_inode_lock);
-    for (auto& watcher : m_watchers) {
+    m_watchers.for_each([&](auto& watcher) {
         watcher->notify_inode_event({}, identifier(), InodeWatcherEvent::Type::ContentModified);
-    }
+    });
 }
 
 void Inode::did_delete_self()
 {
-    MutexLocker locker(m_inode_lock);
-    for (auto& watcher : m_watchers) {
+    m_watchers.for_each([&](auto& watcher) {
         watcher->notify_inode_event({}, identifier(), InodeWatcherEvent::Type::Deleted);
-    }
+    });
 }
 
 ErrorOr<void> Inode::prepare_to_write_data()
@@ -370,4 +366,9 @@ void Inode::remove_flocks_for_description(OpenFileDescription const& description
             m_flocks.remove(i--);
     }
 }
+bool Inode::has_watchers() const
+{
+    return !m_watchers.with([&](auto& watchers) { return watchers.is_empty(); });
+}
+
 }
diff --git a/Kernel/FileSystem/Inode.h b/Kernel/FileSystem/Inode.h
index 1a074c3591b..871f81af972 100644
--- a/Kernel/FileSystem/Inode.h
+++ b/Kernel/FileSystem/Inode.h
@@ -89,7 +89,7 @@ public:
     static void sync_all();
     void sync();
 
-    bool has_watchers() const { return !m_watchers.is_empty(); }
+    bool has_watchers() const;
 
     ErrorOr<void> register_watcher(Badge<InodeWatcher>, InodeWatcher&);
     void unregister_watcher(Badge<InodeWatcher>, InodeWatcher&);
@@ -118,7 +118,7 @@ private:
     InodeIndex m_index { 0 };
     WeakPtr<Memory::SharedInodeVMObject> m_shared_vmobject;
     RefPtr<LocalSocket> m_socket;
-    HashTable<InodeWatcher*> m_watchers;
+    SpinlockProtected<HashTable<InodeWatcher*>> m_watchers;
     bool m_metadata_dirty { false };
     RefPtr<FIFO> m_fifo;
     IntrusiveListNode<Inode> m_inode_list_node;