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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Thu, 17 Sep 2020 09:52:30 +0200
Message-ID: <CAKMK7uFXD7FzGZJZx0QAR2WdbewGmLnsSVaH7+HD0XSr--f0kw@mail.gmail.com>
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ard Biesheuvel <ardb@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
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        Richard Henderson <rth@twiddle.net>,
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On Thu, Sep 17, 2020 at 12:39 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Wed, Sep 16, 2020 at 11:43:02PM +0200, Daniel Vetter wrote:
> > On Wed, Sep 16, 2020 at 10:58 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > >
> > > On Wed, Sep 16, 2020 at 10:29:06PM +0200, Daniel Vetter wrote:
> > > > On Wed, Sep 16, 2020 at 5:29 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > > >
> > > > > On Wed, Sep 16, 2020 at 09:37:17AM +0200, Daniel Vetter wrote:
> > > > > > On Tue, Sep 15, 2020 at 7:35 PM Linus Torvalds
> > > > > > <torvalds@linux-foundation.org> wrote:
> > > > > > >
> > > > > > > On Tue, Sep 15, 2020 at 1:39 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> > > > > > > >
> > > > > > > > OTOH, having a working 'preemptible()' or maybe better named
> > > > > > > > 'can_schedule()' check makes tons of sense to make decisions about
> > > > > > > > allocation modes or other things.
> > > > > > >
> > > > > > > No. I think that those kinds of decisions about actual behavior are
> > > > > > > always simply fundamentally wrong.
> > > > > > >
> > > > > > > Note that this is very different from having warnings about invalid
> > > > > > > use. THAT is correct. It may not warn in all configurations, but that
> > > > > > > doesn't matter: what matters is that it warns in common enough
> > > > > > > configurations that developers will catch it.
> > > > > > >
> > > > > > > So having a warning in "might_sleep()" that doesn't always trigger,
> > > > > > > because you have a limited configuration that can't even detect the
> > > > > > > situation, that's fine and dandy and intentional.
> > > > > > >
> > > > > > > But having code like
> > > > > > >
> > > > > > >        if (can_schedule())
> > > > > > >            .. do something different ..
> > > > > > >
> > > > > > > is fundamentally complete and utter garbage.
> > > > > > >
> > > > > > > It's one thing if you test for "am I in hardware interrupt context".
> > > > > > > Those tests aren't great either, but at least they make sense.
> > > > > > >
> > > > > > > But a driver - or some library routine - making a difference based on
> > > > > > > some nebulous "can I schedule" is fundamentally and basically WRONG.
> > > > > > >
> > > > > > > If some code changes behavior, it needs to be explicit to the *caller*
> > > > > > > of that code.
> > > > > > >
> > > > > > > So this is why GFP_ATOMIC is fine, but "if (!can_schedule())
> > > > > > > do_something_atomic()" is pure shite.
> > > > > > >
> > > > > > > And I am not IN THE LEAST interested in trying to help people doing
> > > > > > > pure shite. We need to fix them. Like the crypto code is getting
> > > > > > > fixed.
> > > > > >
> > > > > > Just figured I'll throw my +1 in from reading too many (gpu) drivers.
> > > > > > Code that tries to cleverly adjust its behaviour depending upon the
> > > > > > context it's running in is harder to understand and blows up in more
> > > > > > interesting ways. We still have drm_can_sleep() and it's mostly just
> > > > > > used for debug code, and I've largely ended up just deleting
> > > > > > everything that used it because when you're driver is blowing up the
> > > > > > last thing you want is to realize your debug code and output can't be
> > > > > > relied upon. Or worse, that the only Oops you have is the one in the
> > > > > > debug code, because the real one scrolled away - the original idea
> > > > > > behind drm_can_sleep was to make all the modeset code work
> > > > > > automagically both in normal ioctl/kworker context and in the panic
> > > > > > handlers or kgdb callbacks. Wishful thinking at best.
> > > > > >
> > > > > > Also at least for me that extends to everything, e.g. I much prefer
> > > > > > explicit spin_lock and spin_lock_irq vs magic spin_lock_irqsave for
> > > > > > locks shared with interrupt handlers, since the former two gives me
> > > > > > clear information from which contexts such function can be called.
> > > > > > Other end is the memalloc_no*_save/restore functions, where I recently
> > > > > > made a real big fool of myself because I didn't realize how much that
> > > > > > impacts everything that's run within - suddenly "GFP_KERNEL for small
> > > > > > stuff never fails" is wrong everywhere.
> > > > > >
> > > > > > It's all great for debugging and sanity checks (and we run with all
> > > > > > that stuff enabled in our CI), but really semantic changes depending
> > > > > > upon magic context checks freak my out :-)
> > > > >
> > > > > All fair, but some of us need to write code that must handle being
> > > > > invoked from a wide variety of contexts.  Now perhaps you like the idea of
> > > > > call_rcu() for schedulable contexts, call_rcu_nosched() when preemption
> > > > > is disabled, call_rcu_irqs_are_disabled() when interrupts are disabled,
> > > > > call_rcu_raw_atomic() from contexts where (for example) raw spinlocks
> > > > > are held, and so on.  However, from what I can see, most people instead
> > > > > consistently prefer that the RCU API instead be consolidated.
> > > > >
> > > > > Some in-flight cache-efficiency work for kvfree_rcu() and call_rcu()
> > > > > needs to be able to allocate memory occasionally.  It can do that when
> > > > > invoked from some contexts, but not when invoked from others.  Right now,
> > > > > in !PREEMPT kernels, it cannot tell, and must either do things to the
> > > > > memory allocators that some of the MM hate or must unnecessarily invoke
> > > > > workqueues.  Thomas's patches would allow the code to just allocate in
> > > > > the common case when these primitives are invoked from contexts where
> > > > > allocation is permitted.
> > > > >
> > > > > If we want to restrict access to the can_schedule() or whatever primitive,
> > > > > fine and good.  We can add a check to checkpatch.pl, for example.  Maybe
> > > > > we can go back to the old brlock approach of requiring certain people's
> > > > > review for each addition to the kernel.
> > > > >
> > > > > But there really are use cases that it would greatly help.
> > > >
> > > > We can deadlock in random fun places if random stuff we're calling
> > > > suddenly starts allocating. Sometimes. Maybe once in a blue moon, to
> > > > make it extra fun to reproduce. Maybe most driver subsystems are less
> > > > brittle, but gpu drivers definitely need to know about the details for
> > > > exactly this example. And yes gpu drivers use rcu for freeing
> > > > dma_fence structures, and that tends to happen in code that we only
> > > > recently figured out should really not allocate memory.
> > > >
> > > > I think minimally you need to throw in an unconditional
> > > > fs_reclaim_acquire();fs_reclaim_release(); so that everyone who runs
> > > > with full debugging knows what might happen. It's kinda like
> > > > might_sleep, but a lot more specific. might_sleep() alone is not
> > > > enough, because in the specific code paths I'm thinking of (and
> > > > created special lockdep annotations for just recently) sleeping is
> > > > allowed, but any memory allocations with GFP_RECLAIM set are no-go.
> > >
> > > Completely agreed!  Any allocation on any free path must be handled
> > > -extremely- carefully.  To that end...
> > >
> > > First, there is always a fallback in case the allocation fails.  Which
> > > might have performance or corner-case robustness issues, but which will
> > > at least allow forward progress.  Second, we consulted with a number of
> > > MM experts to arrive at appropriate GFP_* flags (and their patience is
> > > greatly appreciated).  Third, the paths that can allocate will do so about
> > > one time of 500, so any issues should be spotted sooner rather than later.
> > >
> > > So you are quite right to be concerned, but I believe we will be doing the
> > > right things.  And based on his previous track record, I am also quite
> > > certain that Mr. Murphy will be on hand to provide me any additional
> > > education that I might require.
> > >
> > > Finally, I have noted down your point about fs_reclaim_acquire() and
> > > fs_reclaim_release().  Whether or not they prove to be needed, I do
> > > appreciate your calling them to my attention.
> >
> > I just realized that since these dma_fence structs are refcounted and
> > userspace can hold references (directly, it can pass them around
> > behind file descriptors) we might never hit such a path until slightly
> > unusual or evil userspace does something interesting. Do you have
> > links to those patches? Some googling didn't turn up anything. I can
> > then figure out whether it's better to risk not spotting issues with
> > call_rcu vs slapping a memalloc_noio_save/restore around all these
> > critical section which force-degrades any allocation to GFP_ATOMIC at
> > most, but has the risk that we run into code that assumes "GFP_KERNEL
> > never fails for small stuff" and has a decidedly less tested fallback
> > path than rcu code.
>
> Here is the previous early draft version, which will change considerably
> for the next version:
>
>         lore.kernel.org/lkml/20200809204354.20137-1-urezki@gmail.com
>
> This does kvfree_rcu(), but we expect to handle call_rcu() similarly.
>
> The version in preparation will use workqueues to do the allocation in a
> known-safe environment and also use lockless access to certain portions
> of the allocator caches (as noted earlier, this last is not much loved
> by some of the MM guys).  Given Thomas's patch, we could with high
> probability allocate directly, perhaps even not needing memory-allocator
> modifications.
>
> Either way, kvfree_rcu(), and later call_rcu(), will avoid asking the
> allocator to do anything that the calling context prohibits.  So what
> types of bugs are you looking for?  Where reclaim calls back into the
> driver or some such?

Yeah pretty much. It's a problem for gpu, fs, block drivers and really
anything else that's remotely involved in memory reclaim somehow.
Generally this is all handled explicitly by passing gfp_t flags down
any call chain, but in some cases it's instead solved with the
memalloc_no* functions. E.g. sunrpc uses that to make sure the network
stack (which generally just assumes it can allocate memory) doesn't,
to avoid recursions back into nfs/sunrpc. To my knowledge there's no
way to check at runtime with which gfp flags you're allowed to
allocate memory, a preemptible check is definitely not enough.
Disabled preemption implies only GFP_ATOMIC is allowed (ignoring nmi
and stuff like that), but the inverse is not true.

So if you want the automagic in call_rcu I think either
- we need to replace all explicit gfp flags with the context marking
memalloc_no* across the entire kernel, or at least anywhere rcu might
be used.
- audit all callchains and make sure a call_rcu_noalloc is used
anywhere there might be a problem. probably better to have a
call_rcu_gfp with explicit gfp flags parameter, since generally that
needs to be passed down.

But at least to me the lockless magic in mm sounds a lot safer, since
it contains the complexity and doesn't leak it out to callers of
call_rcu.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch