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Date:   Mon, 4 Nov 2019 08:11:52 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     syzbot <syzbot+08f3e9d26e5541e1ecf2@syzkaller.appspotmail.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>, axboe@kernel.dk,
        justin@coraid.com, linux-block@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs@googlegroups.com
Subject: Re: KCSAN: data-race in process_srcu / synchronize_srcu
Message-ID: <20191104161152.GB20975@paulmck-ThinkPad-P72>
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 <CANpmjNPpvyxZv9N042Uz1gQb7R0B1MOmCa255-czqWsc7SiOxg@mail.gmail.com>
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On Mon, Nov 04, 2019 at 12:31:56PM +0100, Marco Elver wrote:
> +RCU folks, since this is probably a data race in RCU.
> 
> On Mon, 4 Nov 2019 at 12:29, syzbot
> <syzbot+08f3e9d26e5541e1ecf2@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    05f22368 x86, kcsan: Enable KCSAN for x86
> > git tree:       https://github.com/google/ktsan.git kcsan
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17ade7ef600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
> > dashboard link: https://syzkaller.appspot.com/bug?extid=08f3e9d26e5541e1ecf2
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+08f3e9d26e5541e1ecf2@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in process_srcu / synchronize_srcu
> >
> > write to 0xffffffff8604e8a0 of 8 bytes by task 17 on cpu 1:
> >   srcu_gp_end kernel/rcu/srcutree.c:533 [inline]
> >   srcu_advance_state kernel/rcu/srcutree.c:1146 [inline]
> >   process_srcu+0x207/0x780 kernel/rcu/srcutree.c:1237
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> >
> > read to 0xffffffff8604e8a0 of 8 bytes by task 12515 on cpu 0:
> >   srcu_might_be_idle kernel/rcu/srcutree.c:784 [inline]
> >   synchronize_srcu+0x107/0x214 kernel/rcu/srcutree.c:996
> >   fsnotify_connector_destroy_workfn+0x63/0xb0 fs/notify/mark.c:164
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 PID: 12515 Comm: kworker/u4:8 Not tainted 5.4.0-rc3+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Workqueue: events_unbound fsnotify_connector_destroy_workfn
> > ==================================================================
> > Kernel panic - not syncing: panic_on_warn set ...
> > CPU: 0 PID: 12515 Comm: kworker/u4:8 Not tainted 5.4.0-rc3+ #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > Workqueue: events_unbound fsnotify_connector_destroy_workfn
> > Call Trace:
> >   __dump_stack lib/dump_stack.c:77 [inline]
> >   dump_stack+0xf5/0x159 lib/dump_stack.c:113
> >   panic+0x210/0x640 kernel/panic.c:221
> >   kcsan_report.cold+0xc/0x10 kernel/kcsan/report.c:302
> >   __kcsan_setup_watchpoint+0x32e/0x4a0 kernel/kcsan/core.c:411
> >   __tsan_read8 kernel/kcsan/kcsan.c:36 [inline]
> >   __tsan_read8+0x2c/0x30 kernel/kcsan/kcsan.c:36
> >   srcu_might_be_idle kernel/rcu/srcutree.c:784 [inline]
> >   synchronize_srcu+0x107/0x214 kernel/rcu/srcutree.c:996
> >   fsnotify_connector_destroy_workfn+0x63/0xb0 fs/notify/mark.c:164
> >   process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
> >   worker_thread+0xa0/0x800 kernel/workqueue.c:2415
> >   kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
> >   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352
> > Kernel Offset: disabled
> > Rebooting in 86400 seconds..
> >
> >
> > ---
> > This bug is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
> > syzbot will keep track of this bug report. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

Good catch!  Does the patch below help?

							Thanx, Paul

------------------------------------------------------------------------

commit d2ab457602a59353b8853352735dfd094d223480
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Mon Nov 4 08:08:30 2019 -0800

    srcu: Apply *_ONCE() to ->srcu_last_gp_end
    
    The ->srcu_last_gp_end field is accessed from any CPU at any time
    by synchronize_srcu(), so non-initialization references need to use
    READ_ONCE() and WRITE_ONCE().  This commit therefore makes that change.
    
    Reported-by: syzbot+08f3e9d26e5541e1ecf2@syzkaller.appspotmail.com
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d0a9d5b..657e6a7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -530,7 +530,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
 	idx = rcu_seq_state(ssp->srcu_gp_seq);
 	WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
 	cbdelay = srcu_get_delay(ssp);
-	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
+	WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
 	rcu_seq_end(&ssp->srcu_gp_seq);
 	gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
 	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, gpseq))
@@ -762,6 +762,7 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
 	unsigned long flags;
 	struct srcu_data *sdp;
 	unsigned long t;
+	unsigned long tlast;
 
 	/* If the local srcu_data structure has callbacks, not idle.  */
 	local_irq_save(flags);
@@ -780,9 +781,9 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
 
 	/* First, see if enough time has passed since the last GP. */
 	t = ktime_get_mono_fast_ns();
+	tlast = READ_ONCE(ssp->srcu_last_gp_end);
 	if (exp_holdoff == 0 ||
-	    time_in_range_open(t, ssp->srcu_last_gp_end,
-			       ssp->srcu_last_gp_end + exp_holdoff))
+	    time_in_range_open(t, tlast, tlast + exp_holdoff))
 		return false; /* Too soon after last GP. */
 
 	/* Next, check for probable idleness. */