From: David Laight <David.Laight@ACULAB.COM>
To: 'Fenghua Yu' <firstname.lastname@example.org>,
Thomas Gleixner <email@example.com>
Cc: Ingo Molnar <firstname.lastname@example.org>, Borislav Petkov <email@example.com>,
"H Peter Anvin" <firstname.lastname@example.org>,
Paolo Bonzini <email@example.com>,
Dave Hansen <firstname.lastname@example.org>,
Ashok Raj <email@example.com>,
Peter Zijlstra <firstname.lastname@example.org>,
Ravi V Shankar <email@example.com>,
Xiaoyao Li <firstname.lastname@example.org>,
Christopherson Sean J <email@example.com>,
Kalle Valo <firstname.lastname@example.org>,
"Michael Chan" <email@example.com>,
linux-kernel <firstname.lastname@example.org>, x86 <email@example.com>,
Subject: RE: [PATCH v7 15/21] x86/split_lock: Add a sysfs interface to enable/disable split lock detection during run time
Date: Wed, 24 Apr 2019 13:45:38 +0000 [thread overview]
Message-ID: <4a90dd7bf26f44a795ecf55aa54b8ce5@AcuMS.aculab.com> (raw)
From: Fenghua Yu
> Sent: 23 April 2019 21:48
> On Thu, Apr 18, 2019 at 08:41:30AM +0200, Thomas Gleixner wrote:
> > On Wed, 17 Apr 2019, Fenghua Yu wrote:
> > > On Thu, Apr 18, 2019 at 12:47:24AM +0200, Thomas Gleixner wrote:
> > > > On Wed, 17 Apr 2019, Fenghua Yu wrote:
> > > >
> > > > > The interface /sys/device/system/cpu/split_lock_detect is added
> > > > > to allow user to control split lock detection and show current split
> > > > > lock detection setting.
> > > > >
> > > > > Writing [yY1] or [oO][nN] to the file enables split lock detection and
> > > > > writing [nN0] or [oO][fF] disables split lock detection. Split lock
> > > > > detection is enabled or disabled on all CPUs.
> > > > >
> > > > > Reading the file returns current global split lock detection setting:
> > > > > 0: disabled
> > > > > 1: enabled
> > > >
> > > > Again, You explain WHAT this patch does and still there is zero
> > > > justification why this sysfs knob is needed at all. I still do not see any
> > > > reason why this knob should exist.
> > >
> > > An important application has split lock issues which are already discovered
> > > and need to be fixed. But before the issues are fixed, sysadmin still wants to
> > > run the application without rebooting the system, the sysfs knob can be useful
> > > to turn off split lock detection. After the application is done, split lock
> > > detection will be enabled again through the sysfs knob.
> > Are you sure that you are talking about the real world? I might buy the
> > 'off' part somehow, but the 'on' part is beyond theoretical.
> > Even the 'off' part is dubious on a multi user machine. I personally would
> > neither think about using the sysfs knob nor about rebooting the machine
> > simply because I'd consider a lock operation accross a cacheline an malicious
> > DoS attempt. Why would I allow that?
> > So in reality the sysadmin will either move the workload to a machine w/o
> > the #AC magic or just tell the user to fix his crap.
> > > Without the sysfs knob, sysadmin has to reboot the system with kernel option
> > > "no_split_lock_detect" to run the application before the split lock issues
> > > are fixed.
> > >
> > > Is this a valid justification why the sysfs knob is needed? If it is, I can
> > > add the justification in the next version.
> > Why has this information not been in the changelog right away? I'm really
> > tired of asking the same questions and pointing you to
> > Documentation/process over and over.
> So should I remove the sysfs knob patches in the next version?
> Or add the following justification and still keep the sysfs knob patches?
> "To workaround or debug a split lock issue, the administrator may need to
> disable or enable split lock detection during run time without rebooting
> the system."
I've also not seen patches to fix all the places where 'lock bit' operations
get used on int  data.
Testing had showed one structure that needed 'fixing', there are some others
that are in .bss/.data.
A kernel build could suddenly have them misaligned and crossing a cache line.
All the places that cast the pointer to the bit ops are suspect.
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next prev parent reply other threads:[~2019-04-24 13:44 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-17 21:33 [PATCH v7 00/21] x86/split_lock: Enable split lock detection Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 01/21] x86/common: Align cpu_caps_cleared and cpu_caps_set to unsigned long Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 02/21] drivers/net/b44: Align pwol_mask to unsigned long for better performance Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 03/21] wlcore: simplify/fix/optimize reg_ch_conf_pending operations Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 04/21] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access Fenghua Yu
2019-04-18 9:20 ` David Laight
2019-04-18 11:08 ` David Laight
2019-04-18 11:49 ` Thomas Gleixner
2019-04-18 13:14 ` David Laight
2019-04-18 13:26 ` David Laight
2019-04-17 21:33 ` [PATCH v7 05/21] x86/msr-index: Define MSR_IA32_CORE_CAPABILITY and split lock detection bit Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 06/21] x86/cpufeatures: Enumerate MSR_IA32_CORE_CAPABILITY Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 07/21] x86/split_lock: Enumerate split lock detection by MSR_IA32_CORE_CAPABILITY Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 08/21] x86/split_lock: Enumerate split lock detection on Icelake mobile processor Fenghua Yu
2019-04-17 21:33 ` [PATCH v7 09/21] x86/split_lock: Define MSR TEST_CTL register Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 10/21] x86/split_lock: Define per CPU variable to cache MSR TEST_CTL Fenghua Yu
2019-04-17 22:14 ` Thomas Gleixner
2019-04-18 1:28 ` Fenghua Yu
2019-04-18 6:31 ` Thomas Gleixner
2019-04-17 21:34 ` [PATCH v7 11/21] x86/split_lock: Handle #AC exception for split lock Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 12/21] kvm/x86: Emulate MSR IA32_CORE_CAPABILITY Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 13/21] kvm/vmx: Emulate MSR TEST_CTL Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 14/21] x86/split_lock: Enable split lock detection by default Fenghua Yu
2019-04-17 22:41 ` Thomas Gleixner
2019-04-17 21:34 ` [PATCH v7 15/21] x86/split_lock: Add a sysfs interface to enable/disable split lock detection during run time Fenghua Yu
2019-04-17 22:47 ` Thomas Gleixner
2019-04-18 0:57 ` Fenghua Yu
2019-04-18 6:41 ` Thomas Gleixner
2019-04-23 20:48 ` Fenghua Yu
2019-04-24 13:45 ` David Laight [this message]
2019-04-17 21:34 ` [PATCH v7 16/21] x86/split_lock: Document the new sysfs file for split lock detection Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 17/21] x86/clearcpuid: Support multiple clearcpuid options Fenghua Yu
2019-04-17 23:05 ` Thomas Gleixner
2019-04-17 21:34 ` [PATCH v7 18/21] x86/clearcpuid: Support feature flag string in kernel option clearcpuid Fenghua Yu
2019-04-17 23:19 ` Thomas Gleixner
2019-04-17 23:47 ` Fenghua Yu
2019-04-18 6:16 ` Thomas Gleixner
2019-04-17 21:34 ` [PATCH v7 19/21] x86/clearcpuid: Apply cleared feature bits that are forced set before Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 20/21] x86/clearcpuid: Clear CPUID bit in CPUID faulting Fenghua Yu
2019-04-17 21:34 ` [PATCH v7 21/21] x86/clearcpuid: Change document for kernel option clearcpuid Fenghua Yu
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