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Message-ID: <53A70602.8020000@linux.vnet.ibm.com> Date: Sun, 22 Jun 2014 22:06:18 +0530 From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> To: Peter Zijlstra <a.p.zijlstra@...llo.nl> CC: Waiman.Long@...com, tglx@...utronix.de, mingo@...nel.org, linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, xen-devel@...ts.xenproject.org, kvm@...r.kernel.org, paolo.bonzini@...il.com, konrad.wilk@...cle.com, boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com, riel@...hat.com, torvalds@...ux-foundation.org, david.vrabel@...rix.com, oleg@...hat.com, gleb@...hat.com, scott.norton@...com, chegu_vinod@...com, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH 11/11] qspinlock, kvm: Add paravirt support On 06/15/2014 06:17 PM, Peter Zijlstra wrote: > Signed-off-by: Peter Zijlstra<peterz@...radead.org> > --- [...] > + > +void kvm_wait(int *ptr, int val) > +{ > + unsigned long flags; > + > + if (in_nmi()) > + return; > + > + /* > + * Make sure an interrupt handler can't upset things in a > + * partially setup state. > + */ I am seeing hang with even 2 cpu guest (with patches on top of 3.15-rc6 ). looking further with gdb I see one cpu is stuck with native_halt with slowpath flag(_Q_LOCKED_SLOW) set when it was called. (gdb) bt #0 native_halt () at /test/master/arch/x86/include/asm/irqflags.h:55 #1 0xffffffff81033118 in halt (ptr=0xffffffff81eb0e58, val=524291) at /test/master/arch/x86/include/asm/paravirt.h:116 #2 kvm_wait (ptr=0xffffffff81eb0e58, val=524291) at arch/x86/kernel/kvm.c:835 #3 kvm_wait (ptr=0xffffffff81eb0e58, val=524291) at arch/x86/kernel/kvm.c:809 #4 0xffffffff810a2d8e in pv_wait (lock=0xffffffff81eb0e58) at /test/master/arch/x86/include/asm/paravirt.h:744 #5 __pv_wait_head (lock=0xffffffff81eb0e58) at kernel/locking/qspinlock.c:352 Value of lock seem to be 524288 (means already unlocked?) So apart from races Waiman mentioned, are we also in need of smp_mb() here and/or native_queue_unlock()?. Interestingly I see other cpu stuck at multi_cpu_stop(). (gdb) thr 1 [Switching to thread 1 (Thread 1)]#0 multi_cpu_stop (data=0xffff8802140d1da0) at kernel/stop_machine.c:192 192 if (msdata->state != curstate) { Or is it I am missing something. please let me know if .config need to be shared. > + local_irq_save(flags); > + > + /* > + * check again make sure it didn't become free while > + * we weren't looking. > + */ > + if (ACCESS_ONCE(*ptr) != val) > + goto out; > + > + /* > + * halt until it's our turn and kicked. Note that we do safe halt > + * for irq enabled case to avoid hang when lock info is overwritten > + * in irq spinlock slowpath and no spurious interrupt occur to save us. > + */ > + if (arch_irqs_disabled_flags(flags)) > + halt(); > + else > + safe_halt(); > + > +out: > + local_irq_restore(flags); > +} > +#endif /* QUEUE_SPINLOCK */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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