pipe: make poll_usage boolean and annotate its access

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: 3b844826b6c6 ("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>

diff --git a/fs/pipe.c b/fs/pipe.c
index e140ea150bbb14e0ab0c83808e0b155b158bfc6a..d04c3fce28a6455191df0d8945b683e7f8ffabf7 100644 (file)
--- 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 c00c618ef290d726874d18c00c174e71d215a4ae..cb0fd633a6106c099a75298bd7845454c8b4a8ea 100644 (file)
--- 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;