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From: David Laight <[email protected]>
To: 'David Hildenbrand' <[email protected]>,
	Greg KH <[email protected]>
Cc: Al Viro <[email protected]>,
	Nick Desaulniers <[email protected]>,
	Christoph Hellwig <[email protected]>,
	"[email protected]" <[email protected]>,
	Andrew Morton <[email protected]>,
	Jens Axboe <[email protected]>, Arnd Bergmann <[email protected]>,
	David Howells <[email protected]>,
	"[email protected]" 
	<[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
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	<[email protected]>
Subject: RE: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"
Date: Thu, 22 Oct 2020 09:28:13 +0000	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

From: David Hildenbrand
> Sent: 22 October 2020 10:19
> 
> On 22.10.20 11:01, Greg KH wrote:
> > On Thu, Oct 22, 2020 at 10:48:59AM +0200, David Hildenbrand wrote:
> >> On 22.10.20 10:40, David Laight wrote:
> >>> From: David Hildenbrand
> >>>> Sent: 22 October 2020 09:35
> >>>>
> >>>> On 22.10.20 10:26, Greg KH wrote:
> >>>>> On Thu, Oct 22, 2020 at 12:39:14AM +0100, Al Viro wrote:
> >>>>>> On Wed, Oct 21, 2020 at 06:13:01PM +0200, Greg KH wrote:
> >>>>>>> On Fri, Sep 25, 2020 at 06:51:39AM +0200, Christoph Hellwig wrote:
> >>>>>>>> From: David Laight <[email protected]>
> >>>>>>>>
> >>>>>>>> This lets the compiler inline it into import_iovec() generating
> >>>>>>>> much better code.
> >>>>>>>>
> >>>>>>>> Signed-off-by: David Laight <[email protected]>
> >>>>>>>> Signed-off-by: Christoph Hellwig <[email protected]>
> >>>>>>>> ---
> >>>>>>>>  fs/read_write.c | 179 ------------------------------------------------
> >>>>>>>>  lib/iov_iter.c  | 176 +++++++++++++++++++++++++++++++++++++++++++++++
> >>>>>>>>  2 files changed, 176 insertions(+), 179 deletions(-)
> >>>>>>>
> >>>>>>> Strangely, this commit causes a regression in Linus's tree right now.
> >>>>>>>
> >>>>>>> I can't really figure out what the regression is, only that this commit
> >>>>>>> triggers a "large Android system binary" from working properly.  There's
> >>>>>>> no kernel log messages anywhere, and I don't have any way to strace the
> >>>>>>> thing in the testing framework, so any hints that people can provide
> >>>>>>> would be most appreciated.
> >>>>>>
> >>>>>> It's a pure move - modulo changed line breaks in the argument lists
> >>>>>> the functions involved are identical before and after that (just checked
> >>>>>> that directly, by checking out the trees before and after, extracting two
> >>>>>> functions in question from fs/read_write.c and lib/iov_iter.c (before and
> >>>>>> after, resp.) and checking the diff between those.
> >>>>>>
> >>>>>> How certain is your bisection?
> >>>>>
> >>>>> The bisection is very reproducable.
> >>>>>
> >>>>> But, this looks now to be a compiler bug.  I'm using the latest version
> >>>>> of clang and if I put "noinline" at the front of the function,
> >>>>> everything works.
> >>>>
> >>>> Well, the compiler can do more invasive optimizations when inlining. If
> >>>> you have buggy code that relies on some unspecified behavior, inlining
> >>>> can change the behavior ... but going over that code, there isn't too
> >>>> much action going on. At least nothing screamed at me.
> >>>
> >>> Apart from all the optimisations that get rid off the 'pass be reference'
> >>> parameters and strange conditional tests.
> >>> Plenty of scope for the compiler getting it wrong.
> >>> But nothing even vaguely illegal.
> >>
> >> Not the first time that people blame the compiler to then figure out
> >> that something else is wrong ... but maybe this time is different :)
> >
> > I agree, I hate to blame the compiler, that's almost never the real
> > problem, but this one sure "feels" like it.
> >
> > I'm running some more tests, trying to narrow things down as just adding
> > a "noinline" to the function that got moved here doesn't work on Linus's
> > tree at the moment because the function was split into multiple
> > functions.
> >
> > Give me a few hours...
> 
> I might be wrong but
> 
> a) import_iovec() uses:
> - unsigned nr_segs -> int
> - unsigned fast_segs -> int
> b) rw_copy_check_uvector() uses:
> - unsigned long nr_segs -> long
> - unsigned long fast_seg -> long
> 
> So when calling rw_copy_check_uvector(), we have to zero-extend the
> registers used for passing the arguments. That's definitely done when
> calling the function explicitly. Maybe when inlining something is messed up?

That's also not needed on x86-64 - the high bits get cleared by 32bit writes.
But, IIRC, arm64 leaves them unchanged or undefined.

I guessing that every array access uses a *(Rx + Ry) addressing
mode. So indexing an array even with 'unsigned int' requires
an explicit zero-extend on arm64?
(x86-64 ends up with an explicit sign extend when indexing an
array with 'signed int'.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  parent reply	other threads:[~2020-10-22  9:28 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25  4:51 let import_iovec deal with compat_iovecs as well v4 Christoph Hellwig
2020-09-25  4:51 ` [PATCH 1/9] compat.h: fix a spelling error in <linux/compat.h> Christoph Hellwig
2020-09-25  4:51 ` [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c Christoph Hellwig
2020-10-21 16:13   ` Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c" Greg KH
2020-10-21 20:59     ` David Laight
2020-10-21 23:39     ` Al Viro
2020-10-22  8:26       ` Greg KH
2020-10-22  8:35         ` David Hildenbrand
2020-10-22  8:40           ` David Laight
2020-10-22  8:48             ` David Hildenbrand
2020-10-22  9:01               ` Greg KH
2020-10-22  9:06                 ` David Laight
2020-10-22  9:19                 ` David Hildenbrand
2020-10-22  9:25                   ` David Hildenbrand
2020-10-22  9:32                     ` David Laight
2020-10-22  9:36                       ` David Hildenbrand
2020-10-22 10:48                         ` Greg KH
2020-10-22 12:18                           ` Greg KH
2020-10-22 12:42                             ` David Hildenbrand
2020-10-22 12:57                               ` Greg KH
2020-10-22 13:50                                 ` Greg KH
     [not found]                                   ` <CAK8P3a1B7OVdyzW0-97JwzZiwp0D0fnSfyete16QTvPp_1m07A@mail.gmail.com>
2020-10-22 14:40                                     ` Greg KH
2020-10-22 16:15                                       ` David Laight
2020-10-23 12:46                                   ` David Laight
2020-10-23 13:09                                     ` David Hildenbrand
2020-10-23 14:33                                       ` David Hildenbrand
2020-10-23 14:39                                         ` David Laight
2020-10-23 14:47                                           ` 'Greg KH'
2020-10-23 16:33                                             ` David Hildenbrand
2020-11-02  9:06                                             ` David Laight
2020-11-02 13:52                                               ` 'Greg KH'
2020-11-02 18:23                                                 ` David Laight
2020-10-23 17:58                                       ` Al Viro
2020-10-23 18:27                                         ` Segher Boessenkool
2020-10-23 21:28                                           ` David Laight
2020-10-24 17:29                                             ` Segher Boessenkool
2020-10-24 21:12                                               ` David Laight
     [not found]                                     ` <CAK8P3a1n+b8hOMhNQSDzgic03dyXbmpccfTJ3C1bGKvzsgMXbg@mail.gmail.com>
2020-10-23 13:28                                       ` David Laight
2020-10-22 13:23                         ` Christoph Hellwig
2020-10-22 16:35                           ` David Laight
2020-10-22 16:40                             ` Matthew Wilcox
2020-10-22 16:50                               ` David Laight
2020-10-22 17:00                               ` Nick Desaulniers
2020-10-22 20:59                                 ` Eric Biggers
2020-10-22 21:28                                   ` Al Viro
2020-10-22 18:19                               ` Al Viro
2020-10-22 17:54                             ` Nick Desaulniers
     [not found]                               ` <CAK8P3a3LjG+ZvmQrkb9zpgov8xBkQQWrkHBPgjfYSqBKGrwT4w@mail.gmail.com>
2020-10-22 19:04                                 ` Nick Desaulniers
2020-10-22 19:24                                   ` Al Viro
2020-10-22 19:27                                     ` Al Viro
2020-10-22 20:06                                     ` Al Viro
2020-10-22 20:09                                       ` Al Viro
2020-10-22 20:11                                     ` Nick Desaulniers
2020-10-22 22:07                                     ` David Laight
2020-10-23 13:12                                     ` David Hildenbrand
2020-10-22 22:04                                   ` David Laight
2020-10-22  9:28                   ` David Laight [this message]
2020-10-22  9:02               ` David Laight
2020-09-25  4:51 ` [PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec Christoph Hellwig
2020-09-25  4:51 ` [PATCH 4/9] iov_iter: transparently handle compat iovecs in import_iovec Christoph Hellwig
2020-09-25  4:51 ` [PATCH 5/9] fs: remove various compat readv/writev helpers Christoph Hellwig
2020-09-25  4:51 ` [PATCH 6/9] fs: remove the compat readv/writev syscalls Christoph Hellwig
2020-09-25  4:51 ` [PATCH 7/9] fs: remove compat_sys_vmsplice Christoph Hellwig
2020-09-25  4:51 ` [PATCH 8/9] mm: remove compat_process_vm_{readv,writev} Christoph Hellwig
2020-09-25  4:51 ` [PATCH 9/9] security/keys: remove compat_keyctl_instantiate_key_iov Christoph Hellwig
2020-09-25 15:23 ` let import_iovec deal with compat_iovecs as well v4 Al Viro

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