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From: Greg KH <gregkh@linuxfoundation.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>,
	keescook@chromium.org, dhowells@redhat.com, hch@infradead.org,
	mbenes@suse.com, ngupta@vflare.org,
	sergey.senozhatsky.work@gmail.com, axboe@kernel.dk,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] zram: fix crashes due to use of cpu hotplug multistate
Date: Sat, 3 Apr 2021 08:13:23 +0200	[thread overview]
Message-ID: <YGgHg7XCHD3rATIK@kroah.com> (raw)
In-Reply-To: <20210402183016.GU4332@42.do-not-panic.com>

On Fri, Apr 02, 2021 at 06:30:16PM +0000, Luis Chamberlain wrote:
> On Fri, Apr 02, 2021 at 09:54:12AM +0200, Greg KH wrote:
> > On Thu, Apr 01, 2021 at 11:59:25PM +0000, Luis Chamberlain wrote:
> > > As for the syfs deadlock possible with drivers, this fixes it in a generic way:
> > > 
> > > commit fac43d8025727a74f80a183cc5eb74ed902a5d14
> > > Author: Luis Chamberlain <mcgrof@kernel.org>
> > > Date:   Sat Mar 27 14:58:15 2021 +0000
> > > 
> > >     sysfs: add optional module_owner to attribute
> > >     
> > >     This is needed as otherwise the owner of the attribute
> > >     or group read/store might have a shared lock used on driver removal,
> > >     and deadlock if we race with driver removal.
> > >     
> > >     Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> > 
> > No, please no.  Module removal is a "best effort",
> 
> Not for live patching. I am not sure if I am missing any other valid
> use case?

live patching removes modules?  We have so many code paths that are
"best effort" when it comes to module unloading, trying to resolve this
one is a valiant try, but not realistic.

> > if the system dies when it happens, that's on you. 
> 
> I think the better approach for now is simply to call testers / etc to
> deal with this open coded. I cannot be sure that other than live
> patching there may be other valid use cases for module removal, and for
> races we really may care for where userspace *will* typically be mucking
> with sysfs attributes. Monitoring my systems's sysfs attributes I am
> actually quite surprised at the random pokes at them.
> 
> > I am not willing to expend extra energy
> > and maintance of core things like sysfs for stuff like this that does
> > not matter in any system other than a developer's box.
> 
> Should we document this as well? Without this it is unclear that tons of
> random tests are sanely nullified. At least this dead lock I spotted can
> be pretty common form on many drivers.

What other drivers have this problem?

> > Lock data, not code please.  Trying to tie data structure's lifespans
> > to the lifespan of code is a tangled mess, and one that I do not want to
> > add to in any form.
> 
> Driver developers will simply have to open code these protections. In
> light of what I see on LTP / fuzzing, I suspect the use case will grow
> and we'll have to revisit this in the future. But for now, sure, we can
> just open code the required protections everywhere to not crash on module
> removal.

LTP and fuzzing too do not remove modules.  So I do not understand the
root problem here, that's just something that does not happen on a real
system.

thanks,

greg k-h

  reply	other threads:[~2021-04-03  6:13 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-06  2:20 [PATCH 0/2] zram: fix few ltp zram02.sh crashes Luis Chamberlain
2021-03-06  2:20 ` [PATCH 1/2] zram: fix crashes due to use of cpu hotplug multistate Luis Chamberlain
2021-03-09  2:55   ` Minchan Kim
2021-03-10 13:11     ` Luis Chamberlain
2021-03-10 21:25       ` Luis Chamberlain
2021-03-12  2:08       ` Minchan Kim
2021-03-10 21:21     ` Luis Chamberlain
2021-03-12  2:14       ` Minchan Kim
2021-03-12 18:32         ` Luis Chamberlain
2021-03-12 19:28           ` Minchan Kim
2021-03-19 19:09             ` Luis Chamberlain
2021-03-22 16:37               ` Minchan Kim
2021-03-22 20:41                 ` Luis Chamberlain
2021-03-22 22:12                   ` Minchan Kim
2021-04-01 23:59                     ` Luis Chamberlain
2021-04-02  7:54                       ` Greg KH
2021-04-02 18:30                         ` Luis Chamberlain
2021-04-03  6:13                           ` Greg KH [this message]
     [not found]                             ` <20210406003152.GZ4332@42.do-not-panic.com>
2021-04-06 12:00                               ` Miroslav Benes
2021-04-06 15:54                                 ` Josh Poimboeuf
2021-04-07 14:09                                   ` Peter Zijlstra
2021-04-07 15:30                                     ` Josh Poimboeuf
2021-04-07 16:48                                       ` Peter Zijlstra
2021-04-07 20:17                         ` Josh Poimboeuf
2021-04-08  6:18                           ` Greg KH
2021-04-08 13:16                             ` Steven Rostedt
2021-04-08 13:37                             ` Josh Poimboeuf
2021-04-08  1:37                         ` Thomas Gleixner
2021-04-08  6:16                           ` Greg KH
2021-04-08  8:01                             ` Jiri Kosina
2021-04-08  8:09                               ` Greg KH
2021-04-08  8:35                                 ` Jiri Kosina
2021-04-08  8:55                                   ` Greg KH
2021-04-08 18:40                                     ` Luis Chamberlain
2021-04-09  3:01                                     ` Kees Cook
2021-04-05 17:07                       ` Minchan Kim
2021-04-05 19:00                         ` Luis Chamberlain
2021-04-05 19:58                           ` Minchan Kim
2021-04-06  0:29                             ` Luis Chamberlain
2021-04-07  1:23                               ` Minchan Kim
2021-04-07  1:38                                 ` Minchan Kim
2021-04-07 14:52                                   ` Luis Chamberlain
2021-04-07 14:50                                 ` Luis Chamberlain
2021-03-06  2:20 ` [PATCH 2/2] zram: fix races of sysfs attribute removal and usage Luis Chamberlain

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