From: Dave Hansen <dave.hansen@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Qiaowei Ren <qiaowei.ren@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-ia64@vger.kernel.org, linux-mips@linux-mips.org
Subject: Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables
Date: Tue, 28 Oct 2014 10:43:48 -0700 [thread overview]
Message-ID: <544FD5D4.4090404@intel.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1410241257300.5308@nanos>
On 10/24/2014 05:08 AM, Thomas Gleixner wrote:
> On Sun, 12 Oct 2014, Qiaowei Ren wrote:
>> + /*
>> + * Go poke the address of the new bounds table in to the
>> + * bounds directory entry out in userspace memory. Note:
>> + * we may race with another CPU instantiating the same table.
>> + * In that case the cmpxchg will see an unexpected
>> + * 'actual_old_val'.
>> + */
>> + ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry,
>> + expected_old_val, bt_addr);
>
> This is fully preemptible non-atomic context, right?
>
> So this wants a proper comment, why using
> user_atomic_cmpxchg_inatomic() is the right thing to do here.
Hey Thomas,
How's this for a new comment? Does this cover the points you think need
clarified?
====
The kernel has allocated a bounds table and needs to point the
(userspace-allocated) directory to it. The directory entry is the
*only* place we track that this table was allocated, so we essentially
use it instead of an kernel data structure for synchronization. A
copy_to_user()-style function would not give us the atomicity that we need.
If two threads race to instantiate a table, the cmpxchg ensures we know
which one lost the race and that the loser frees the table that they
just allocated.
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next prev parent reply other threads:[~2014-10-28 17:43 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-12 4:41 [PATCH v9 00/12] Intel MPX support Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 01/12] x86, mpx: introduce VM_MPX to indicate that a VMA is MPX specific Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 02/12] x86, mpx: rename cfg_reg_u and status_reg Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 03/12] x86, mpx: add MPX specific mmap interface Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 04/12] x86, mpx: add MPX to disaabled features Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables Qiaowei Ren
2014-10-24 12:08 ` Thomas Gleixner
2014-10-27 3:20 ` Ren Qiaowei
2014-10-28 17:43 ` Dave Hansen [this message]
2014-10-28 17:57 ` Thomas Gleixner
2014-10-12 4:41 ` [PATCH v9 06/12] mpx: extend siginfo structure to include bound violation information Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 07/12] mips: sync struct siginfo with general version Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 08/12] ia64: " Qiaowei Ren
2014-10-12 4:41 ` [PATCH v9 09/12] x86, mpx: decode MPX instruction to get bound violation information Qiaowei Ren
2014-10-24 12:36 ` Thomas Gleixner
2014-10-27 1:43 ` Ren, Qiaowei
2014-10-27 20:36 ` Thomas Gleixner
2014-10-28 5:58 ` Ren Qiaowei
2014-10-31 20:16 ` Dave Hansen
2014-10-31 20:33 ` Thomas Gleixner
2014-10-30 22:38 ` Dave Hansen
2014-10-31 2:12 ` Ren Qiaowei
2014-10-31 9:09 ` Thomas Gleixner
2014-10-12 4:41 ` [PATCH v9 10/12] x86, mpx: add prctl commands PR_MPX_ENABLE_MANAGEMENT, PR_MPX_DISABLE_MANAGEMENT Qiaowei Ren
2014-10-24 12:49 ` Thomas Gleixner
2014-10-24 15:10 ` Thomas Gleixner
2014-10-27 2:17 ` Ren, Qiaowei
2014-10-27 20:38 ` Thomas Gleixner
2014-10-28 5:57 ` Ren Qiaowei
2014-10-12 4:41 ` [PATCH v9 11/12] x86, mpx: cleanup unused bound tables Qiaowei Ren
2014-10-24 14:40 ` Thomas Gleixner
2014-10-27 3:13 ` Ren Qiaowei
2014-10-27 20:49 ` Thomas Gleixner
2014-10-28 5:56 ` Ren Qiaowei
2014-10-28 10:42 ` Thomas Gleixner
2014-11-03 20:53 ` Dave Hansen
2014-11-03 21:29 ` Thomas Gleixner
2014-11-04 16:00 ` Dave Hansen
2014-11-04 17:02 ` Thomas Gleixner
2014-11-06 21:50 ` Dave Hansen
2014-11-11 18:27 ` Thomas Gleixner
2014-11-11 20:44 ` Dave Hansen
2014-11-11 21:36 ` Thomas Gleixner
2014-10-12 4:41 ` [PATCH v9 12/12] x86, mpx: add documentation on Intel MPX Qiaowei Ren
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