All of lore.kernel.org
 help / color / mirror / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: David Laight <David.Laight@aculab.com>,
	"sedat.dilek@gmail.com" <sedat.dilek@gmail.com>,
	Jiri Kosina <jikos@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>, Tejun Heo <tj@kernel.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Benjamin Tissoires <benjamin.tissoires@redhat.com>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Andy Lutomirski <luto@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	USB list <linux-usb@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH] usbhid: Fix lockdep unannotated irqs-off warning
Date: Mon, 7 Mar 2016 10:30:16 -0800	[thread overview]
Message-ID: <CA+55aFxQdV1bPmfSMXmH9HRqhmP4g79Lp=XMYqNqZ+SxJtH=fA@mail.gmail.com> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1603071303070.1611-100000@iolanthe.rowland.org>

On Mon, Mar 7, 2016 at 10:07 AM, Alan Stern <stern@rowland.harvard.edu> wrote:
>
> Of course, there are other ways to save a single flag value (such as
> setz).  It's up to the compiler developers to decide what they think is
> best.

Using 'setcc' to save eflags somewhere is definitely the right thing to do.

Using pushf/popf in generated code is completely insane (unless done
very localized in a controlled area).

It is, in fact, insane and wrong even in user space, since eflags does
contain bits that user space itself might be modifying.

In fact, even IF may be modified with iopl 3 (thing old X server
setups), but ignoring that flag entirely, you have AC that acts in
very similar ways (system-wide alignment control) that user space
might be using to make sure it doesn't have unaligned accesses.

It's rare, yes. But still - this isn't really limited to just the kernel.

But perhaps more importantly, I suspect using pushf/popf isn't just
semantically the wrong thing to do, it's just plain stupid. It's
likely slower than the obvious 'setcc' model. Agner Fog's table shows
it "popf" as being 25-30 uops on several microarchitectures. Looks
like it's often microcode.

Now, pushf/popf may well be fairly cheap on *some* uarchitectures, but
it really sounds like a bad idea to use it when not absolutely
required. And that is completely independent of the fact that is
screws up the IF bit.

But yeah, for the kernel we at a minimum need a way to disable that
code generation, even if the clang guys might have some insane reason
to keep it for other cases.

              Linus

  reply	other threads:[~2016-03-07 18:30 UTC|newest]

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-07 15:59 [PATCH] usbhid: Fix lockdep unannotated irqs-off warning Sedat Dilek
2016-03-07 16:28 ` Sedat Dilek
2016-03-07 16:41 ` Alan Stern
2016-03-07 17:03   ` Steven Rostedt
2016-03-07 17:05   ` Jiri Kosina
2016-03-07 17:15     ` Sedat Dilek
2016-03-07 17:27       ` David Laight
2016-03-07 18:07         ` Alan Stern
2016-03-07 18:30           ` Linus Torvalds [this message]
2016-06-27 19:50             ` Sedat Dilek
2016-06-27 20:03               ` Sedat Dilek
2016-06-27 20:14               ` Linus Torvalds
2016-06-27 20:27                 ` Sedat Dilek
2016-06-27 20:36                   ` Linus Torvalds
2016-03-07 17:11   ` Steven Rostedt
2016-03-07 17:18   ` Sedat Dilek
2016-03-07 17:24     ` Jiri Kosina
2016-03-07 17:30       ` Steven Rostedt
2016-03-07 18:04         ` Andy Lutomirski
2016-03-07 19:10           ` Steven Rostedt
  -- strict thread matches above, loose matches on Subject: below --
2015-09-28  8:10 Sedat Dilek
2015-09-28 11:33 ` Jiri Kosina
2015-09-29  8:40   ` Sedat Dilek
2015-09-29  9:27     ` Jiri Kosina
2015-09-29 18:54       ` Sedat Dilek
2015-09-29 19:08         ` Steven Rostedt
2015-09-29 19:32           ` Sedat Dilek
     [not found]       ` <CA+icZUWH2vR_vpYu4hCS578U3ssmoiF0pLYUfM-Xo-57e8uN=g@mail.gmail.com>
2015-09-30  6:41         ` Sedat Dilek
2015-09-30  8:50           ` Steven Rostedt
2015-09-30  7:35         ` Jiri Kosina
2015-09-30  8:56           ` Steven Rostedt
2015-09-30  9:46             ` Sedat Dilek
2015-09-30 10:13               ` Steven Rostedt
2015-09-30 10:39                 ` Sedat Dilek
     [not found]                 ` <CA+icZUXSzScTmMgLZwPQq9RMH9cUsD5_iDxKTVuG0rrGqH-8Cw@mail.gmail.com>
2015-10-01  2:01                   ` Steven Rostedt
2015-10-01  5:34                     ` Sedat Dilek
2015-10-01  6:05                     ` Sedat Dilek
     [not found]                       ` <CA+icZUUyaHqHP2v52juhGhoTNS9xX7LT2YxkOppLz6f9Z+FBEA@mail.gmail.com>
     [not found]                         ` <CA+icZUWagGMVNs5gBPRBhYO0LsY2A1hK3KSLabp9ZpDVOTmtig@mail.gmail.com>
2015-10-13  0:57                           ` Steven Rostedt
2016-03-01 10:05                             ` Sedat Dilek
2016-03-01 15:07                               ` Steven Rostedt
2016-03-01 15:17                                 ` Peter Zijlstra
2016-03-02 15:00                                   ` Sedat Dilek
2016-03-02 15:17                                     ` Peter Zijlstra
2016-03-02 15:34                                       ` Sedat Dilek
2016-03-02 15:53                                       ` Sedat Dilek
2016-03-02 15:56                                         ` Steven Rostedt
2016-03-02 16:08                                           ` Sedat Dilek
2016-03-02 16:11                                             ` Sedat Dilek
2016-03-02 16:21                                               ` Sedat Dilek
2016-03-02 16:24                                         ` Peter Zijlstra
2016-03-02 16:35                                           ` Steven Rostedt
2016-03-02 16:42                                             ` Peter Zijlstra
2016-03-02 16:42                                           ` Sedat Dilek
2016-03-02 16:52                                             ` Sedat Dilek
2016-03-01 15:59                               ` Alan Stern
2016-03-02  6:25                                 ` Sedat Dilek
2016-03-04 16:04                                   ` Alan Stern
2016-03-05 22:30                                     ` Sedat Dilek
2016-03-06  8:51                                       ` Sedat Dilek
2016-03-06 17:23                                       ` Alan Stern
2016-03-02  6:36                                 ` Sedat Dilek
2016-03-02  8:34                                   ` Sedat Dilek
2016-03-02  8:37                                     ` Jiri Kosina
2016-03-02  9:11                                       ` Sedat Dilek
     [not found]     ` <CA+icZUX3tJvRor6CbOZFBecTAUZzyWzjLzJSEdb3c12yKRAT3g@mail.gmail.com>
2015-09-29 13:13       ` Steven Rostedt

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CA+55aFxQdV1bPmfSMXmH9HRqhmP4g79Lp=XMYqNqZ+SxJtH=fA@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=David.Laight@aculab.com \
    --cc=benjamin.tissoires@redhat.com \
    --cc=gregkh@linuxfoundation.org \
    --cc=jiangshanlai@gmail.com \
    --cc=jikos@kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-usb@vger.kernel.org \
    --cc=luto@kernel.org \
    --cc=mingo@redhat.com \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=sedat.dilek@gmail.com \
    --cc=stern@rowland.harvard.edu \
    --cc=tj@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.