Re: [PATCH] context_tracking: Restore previous state in schedule_user
From: Frederic Weisbecker
Date: Wed Dec 03 2014 - 18:58:45 EST
On Wed, Dec 03, 2014 at 03:18:41PM -0800, Andy Lutomirski wrote:
> It appears that some SCHEDULE_USER (asm for schedule_user) callers
> in arch/x86/kernel/entry_64.S are called from RCU kernel context,
> and schedule_user will return in RCU user context. This causes RCU
> warnings and possible failures.
> This is intended to be a minimal fix suitable for 3.18.
> Reported-by: Dave Jones <davej@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Frédéric Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Ah, we sent it about at the same time :-)
Might be too late for 3.18 though because it's not a regression.
> Hi all-
> This is intended to be a suitable last-minute fix for the RCU issue that
> Dave saw.
> Dave, can you confirm that this fixes it?
> Frédéric, can you confirm that you think that this will have no effect
> on correct callers of schedule_user and that will do the right thing
> for incorrect callers of schedule_user?
Yes it should be fine.
> I don't like the x86 asm that calls this at all, and I don't really
> like the fragility of the mechanism is general, but I think that this
> improves the situation enough to avoid problems in the short term.
At best we should have only one call to user_enter() at the end of the
syscall and exception path once we've completed everything (pending reschedule,
tracing, signals, ...) instead of context tracking fixups on functions that
can be called after syscall_trace_leave(), but that would impact the fastpath.
Although it should be possible to tweak the slow path to do that...
> With the obvious warning added, I get:
> [ 0.751022] ------------[ cut here ]------------
> [ 0.751937] WARNING: CPU: 0 PID: 72 at kernel/sched/core.c:2883 schedule_user+0xcf/0xe0()
> [ 0.753477] Modules linked in:
> [ 0.754089] CPU: 0 PID: 72 Comm: mount Not tainted 3.18.0-rc7+ #653
> [ 0.755258] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
> [ 0.757655] 0000000000000009 ffff880005c13f00 ffffffff81741dca ffff8800069f5a50
> [ 0.759228] 0000000000000000 ffff880005c13f40 ffffffff8108e781 0000000000000246
> [ 0.760758] 0000000000000000 00007fff970441c8 00007fff97043fd0 00007f67794ebcc8
> [ 0.762294] Call Trace:
> [ 0.762775] [<ffffffff81741dca>] dump_stack+0x46/0x58
> [ 0.763739] [<ffffffff8108e781>] warn_slowpath_common+0x81/0xa0
> [ 0.764865] [<ffffffff8108e85a>] warn_slowpath_null+0x1a/0x20
> [ 0.765958] [<ffffffff8174565f>] schedule_user+0xcf/0xe0
> [ 0.766974] [<ffffffff8174ae69>] sysret_careful+0x19/0x1c
> [ 0.768011] ---[ end trace 329f34db2b3be966 ]---
> So, yes, we have a bug, and this could cause any number of strange
> kernel/sched/core.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 24beb9bb4c3e..39d9d95331b7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2874,10 +2874,14 @@ asmlinkage __visible void __sched schedule_user(void)
> * or we have been woken up remotely but the IPI has not yet arrived,
> * we haven't yet exited the RCU idle mode. Do it here manually until
> * we find a better solution.
Just need to fix the above comment.
> + *
> + * NB: There are buggy callers of this function. Ideally we
> + * should warn if prev_state != IN_USER, but that will trigger
> + * to frequently to make sense yet.
It's not really the callers of this function that are buggy but the
way we handled context tracking.
> - user_exit();
> + enum ctx_state prev_state = exception_enter();
> - user_enter();
> + exception_exit(prev_state);
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