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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 22 Apr 2019 09:23:53 -0700
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Subject: Re: WARNING in percpu_ref_kill_and_confirm
To:     syzbot <syzbot+10d25e23199614b7721f@syzkaller.appspotmail.com>
Cc:     Arnd Bergmann <arnd@arndb.de>, Jens Axboe <axboe@kernel.dk>,
        Borislav Petkov <bp@alien8.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Anvin <hpa@zytor.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
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        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@redhat.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
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On Mon, Apr 22, 2019 at 9:06 AM syzbot
<syzbot+10d25e23199614b7721f@syzkaller.appspotmail.com> wrote:
>
>
> The bug was bisected to:
>
> commit 38e7571c07be01f9f19b355a9306a4e3d5cb0f5b
> Author: Linus Torvalds <torvalds@linux-foundation.org>
> Date:   Fri Mar 8 22:48:40 2019 +0000
>
>      Merge tag 'io_uring-2019-03-06' of git://git.kernel.dk/linux-block
>
> percpu_ref_kill_and_confirm called more than once on io_ring_ctx_ref_free!

So I don't see how that happens in the original code (because
__io_uring_register() is called with the uring_lock held), but let's
see.

HOWEVER.

I do see how it happens now as of the latest kernel as of commit
b19062a56726 ("io_uring: fix possible deadlock between
io_uring_{enter,register}") where the code explicitly drops the mutex
in order to wait for other uring users to finish.

So Jens, I think that commit was buggy. I suspect that
io_uring_register() should perhaps do something like

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2934,7 +2934,10 @@ static int __io_uring_register(struct
io_ring_ctx *ctx, unsigned opcode,
 {
        int ret;

+       if (!percpu_ref_tryget(&ctx->refs))
+               return -EBUSY;
        percpu_ref_kill(&ctx->refs);
+       percpu_ref_put(&ctx->refs);

        /*
         * Drop uring mutex before waiting for references to exit. If another

to guarantee that it's the *only* case of io_uring_register() doing that kill.

Hmm?

                Linus