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pipe: make poll_usage boolean and annotate its access

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Patch series "Fix data-races around epoll reported by KCSAN."

This series suppresses a false positive KCSAN's message and fixes a real
data-race.


This patch (of 2):

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

Link: https://lkml.kernel.org/r/20220322002653.33865-1-kuniyu@amazon.co.jp
Link: https://lkml.kernel.org/r/20220322002653.33865-2-kuniyu@amazon.co.jp
Fixes: 3b844826b6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kuniyuki Iwashima <kuni1840@gmail.com>
Cc: "Soheil Hassas Yeganeh" <soheil@google.com>
Cc: "Sridhar Samudrala" <sridhar.samudrala@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Kuniyuki Iwashima 2022-04-29 14:38:01 -07:00 committed by akpm
parent d1bd5fa076
commit f485922d8f
2 changed files with 2 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
fs/pipe.c
View File

@ -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.

2
include/linux/pipe_fs_i.h
View File

@ -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;