From: Dave Hansen <dave.hansen@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>
Cc: Jike Song <albcamus@gmail.com>,
linux-kernel@vger.kernel.org, David Woodhouse <dwmw@amazon.co.uk>,
Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
Jiri Koshina <jikos@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Tim Chen <tim.c.chen@linux.intel.com>,
Andi Lutomirski <luto@amacapital.net>,
Andi Kleen <ak@linux.intel.com>,
Peter Zijlstra <peterz@infradead.org>,
Paul Turner <pjt@google.com>,
Tom Lendacky <thomas.lendacky@amd.com>,
Greg KH <gregkh@linux-foundation.org>,
Kees Cook <keescook@google.com>,
stable@vger.kernel.org
Subject: Re: [PATCH v2] x86/mm/pti: remove dead logic during user pagetable population
Date: Sun, 7 Jan 2018 10:34:11 -0800 [thread overview]
Message-ID: <58af26cd-dd39-a544-6449-e59d485bc551@intel.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1801071303540.2094@nanos>
On 01/07/2018 04:05 AM, Thomas Gleixner wrote:
>> static DEFINE_SPINLOCK(shadow_table_allocation_lock);
>>
>> Now I have my suspicions why that's not needed anymore upstream but I'd
>> let tglx explain better.
> We got rid of all that runtime mapping stuff and the functions are only
> called from pti_init(). So the locking and therefor the tests above are not
> needed anymore. While at it we should mark all those function __init.
Yes, the double-test was part of an optimization where we attempted to
avoid using a global spinlock in the fork() path. We would check for
unallocated mid-level page tables without the lock. The lock was only
taken it when we needed to *make* an entry to avoid collisions.
Now that it is all single-threaded, there is no chance of a collision,
no need for a lock, and no need for the re-check.
^^ Just in case someone wants to include a bit more first-hand
information about wtf that code was doing there.
next prev parent reply other threads:[~2018-01-07 18:34 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-07 10:33 Jike Song
2018-01-07 11:36 ` Borislav Petkov
2018-01-07 12:05 ` Thomas Gleixner
2018-01-07 18:34 ` Dave Hansen [this message]
2018-01-08 16:03 ` [PATCH v3] " Jike Song
2018-01-08 20:43 ` [tip:x86/pti] x86/mm/pti: Remove dead logic in pti_user_pagetable_walk*() tip-bot for Jike Song
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