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From:   Kuniyuki Iwashima <kuniyu@amazon.co.jp>
To:     Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     Kuniyuki Iwashima <kuniyu@amazon.co.jp>,
        Kuniyuki Iwashima <kuni1840@gmail.com>,
        <linux-fsdevel@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 1/2] pipe: Make poll_usage boolean and annotate its access.
Date:   Tue, 22 Mar 2022 09:26:52 +0900
Message-ID: <20220322002653.33865-2-kuniyu@amazon.co.jp>
In-Reply-To: <20220322002653.33865-1-kuniyu@amazon.co.jp>
References: <20220322002653.33865-1-kuniyu@amazon.co.jp>
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pipe_poll() runs locklessly and assigns 1 to poll_usage.  Once poll_usage
is set to 1, it never changes in other places.  However, concurrent writes
of a value trigger KCSAN, so let's make KCSAN happy.

BUG: KCSAN: data-race in pipe_poll / pipe_poll

write to 0xffff8880042f6678 of 4 bytes by task 174 on cpu 3:
 pipe_poll (fs/pipe.c:656)
 ep_item_poll.isra.0 (./include/linux/poll.h:88 fs/eventpoll.c:853)
 do_epoll_wait (fs/eventpoll.c:1692 fs/eventpoll.c:1806 fs/eventpoll.c:2234)
 __x64_sys_epoll_wait (fs/eventpoll.c:2246 fs/eventpoll.c:2241 fs/eventpoll.c:2241)
 do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:113)

write to 0xffff8880042f6678 of 4 bytes by task 177 on cpu 1:
 pipe_poll (fs/pipe.c:656)
 ep_item_poll.isra.0 (./include/linux/poll.h:88 fs/eventpoll.c:853)
 do_epoll_wait (fs/eventpoll.c:1692 fs/eventpoll.c:1806 fs/eventpoll.c:2234)
 __x64_sys_epoll_wait (fs/eventpoll.c:2246 fs/eventpoll.c:2241 fs/eventpoll.c:2241)
 do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:113)

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 177 Comm: epoll_race Not tainted 5.17.0-58927-gf443e374ae13 #6
Hardware name: Red Hat KVM, BIOS 1.11.0-2.amzn2 04/01/2014

Fixes: 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
---
CC: Linus Torvalds <torvalds@linux-foundation.org>

Note that the message is false positive for now, so the Fixes tag might not
be necessary.
---
 fs/pipe.c                 | 2 +-
 include/linux/pipe_fs_i.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 9648ac151..e9f8290f8 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -653,7 +653,7 @@ pipe_poll(struct file *filp, poll_table *wait)
 	unsigned int head, tail;
 
 	/* Epoll has some historical nasty semantics, this enables them */
-	pipe->poll_usage = 1;
+	WRITE_ONCE(pipe->poll_usage, true);
 
 	/*
 	 * Reading pipe state only -- no need for acquiring the semaphore.
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index c00c618ef..cb0fd633a 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -71,7 +71,7 @@ struct pipe_inode_info {
 	unsigned int files;
 	unsigned int r_counter;
 	unsigned int w_counter;
-	unsigned int poll_usage;
+	bool poll_usage;
 	struct page *tmp_page;
 	struct fasync_struct *fasync_readers;
 	struct fasync_struct *fasync_writers;
-- 
2.30.2