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