On Thu, Jun 28, 2018 at 09:53:17AM +0200, Thomas Gleixner wrote: > On Wed, 27 Jun 2018, Fenghua Yu wrote: > > On Thu, Jun 21, 2018 at 09:55:40PM +0200, Peter Zijlstra wrote: > > > On Sun, May 27, 2018 at 08:45:53AM -0700, Fenghua Yu wrote: > > > > set_bit() called by set_cpu_cap() is a locked bit set instruction for > > > > atomic operation. > > > > > > > > Since the c->x86_capability can span two cache lines depending on kernel > > > > configuration and building evnironment, the locked bit set instruction > > > > may > > > > cause #AC exception when #AC exception for split lock is enabled. > > > > > > That doesn't make sense. Sure the bitmap may be longer, but depending on > > > if the argument is an immediate or not we either use a byte instruction > > > (which can never cross a cacheline boundary) or a 'word' aligned BTS. > > > And the bitmap really _should_ be 'unsigned long' aligned. > > > > > > If it is not aligned, fix that too. > > > > > > /me looks at cpuinfo_x86 and finds x86_capability is in fact a __u32 > > > array.. see that's broken and needs fixing first. > > > > Do you mean x86_capability's type should be changed from __u32 to unsigned > > long? > > > > Changing x86_capability's type won't directly fix the split lock in > > set_cpu_cap(), right? BTS still may access x86_capability across cache > > line no matter x86_capability's type. > > Errm. No. BTS & al are accessing a single 64bit location which is > > base_address + (bit_offset % 64) * 8 > > So if the base address is properly aligned then BTS & al will _NEVER_ have > to lock more than a single cache line. And it does not matter wheter we fix > the type or enforce 64bit alignement of the array by other means. > > If that's not true then BTS & al are terminally broken and you can stop > working on #AC right away.
Is the following patch right fix for the x86_capability split lock issue (i.e. this patch replace patch 4 and 5)? - __u32 x86_capability[NCAPINTS + NBUGINTS]; + __u32 x86_capability[NCAPINTS + NBUGINTS] + __aligned(8); Thanks. -Fenghua
