Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

On 02/07/2015 12:27 AM, Sasha Levin wrote:
> On 02/06/2015 09:49 AM, Raghavendra K T wrote:
> > Paravirt spinlock clears slowpath flag after doing unlock.
> > As explained by Linus currently it does:
> >                  prev = *lock;
> >                  add_smp(&lock->tickets.head, TICKET_LOCK_INC);
> >
> >                  /* add_smp() is a full mb() */
> >
> >                  if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
> >                          __ticket_unlock_slowpath(lock, prev);
> >
> >
> > which is *exactly* the kind of things you cannot do with spinlocks,
> > because after you've done the "add_smp()" and released the spinlock
> > for the fast-path, you can't access the spinlock any more.  Exactly
> > because a fast-path lock might come in, and release the whole data
> > structure.
> >
> > Linus suggested that we should not do any writes to lock after unlock(),
> > and we can move slowpath clearing to fastpath lock.
> >
> > However it brings additional case to be handled, viz., slowpath still
> > could be set when somebody does arch_trylock. Handle that too by ignoring
> > slowpath flag during lock availability check.
> >
> > Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> With this patch, my VMs lock up quickly after boot with:
Tried to reproduce the hang myself, and there seems to be still a
barrier (or logic I miss).

Looking closely below, unlock_kick got missed though we see
that SLOWPATH_FLAG is still set:

/me goes back to look closely

(gdb) bt
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
#1  0xffffffff81037c27 in halt () at ./arch/x86/include/asm/paravirt.h:116
#2  kvm_lock_spinning (lock=0xffff88023ffe8240, want=52504) at 
arch/x86/kernel/kvm.c:786
#3  0xffffffff81037251 in __raw_callee_save_kvm_lock_spinning ()
#4  0xffff88023fc0edb0 in ?? ()
#5  0x0000000000000000 in ?? ()

(gdb) p *(arch_spinlock_t *)0xffff88023ffe8240
$1 = {{head_tail = 3441806612, tickets = {head = 52500, tail = 52517}}}
(gdb) t 2
[Switching to thread 2 (Thread 2)]
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
55      }
(gdb) bt
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
#1  0xffffffff81037c27 in halt () at ./arch/x86/include/asm/paravirt.h:116
#2  kvm_lock_spinning (lock=0xffff88023ffe8240, want=52502) at 
arch/x86/kernel/kvm.c:786
#3  0xffffffff81037251 in __raw_callee_save_kvm_lock_spinning ()
#4  0x0000000000000246 in irq_stack_union ()
#5  0x0000000000080750 in ?? ()
#6  0x0000000000020000 in ?? ()
#7  0x0000000000000004 in irq_stack_union ()
#8  0x000000000000cd16 in nmi_print_seq ()
Cannot access memory at address 0xbfc0
(gdb) t 3
[Switching to thread 3 (Thread 3)]
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
55      }
(gdb) bt
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
#1  0xffffffff81037c27 in halt () at ./arch/x86/include/asm/paravirt.h:116
#2  kvm_lock_spinning (lock=0xffff88023ffe8240, want=52512) at 
arch/x86/kernel/kvm.c:786
#3  0xffffffff81037251 in __raw_callee_save_kvm_lock_spinning ()
#4  0xffff88023fc8edb0 in ?? ()
#5  0x0000000000000000 in ?? ()

[...] //other threads with similar output

(gdb) t 8
[Switching to thread 8 (Thread 8)]
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
55      }
(gdb) bt
#0  native_halt () at ./arch/x86/include/asm/irqflags.h:55
#1  0xffffffff81037c27 in halt () at ./arch/x86/include/asm/paravirt.h:116
#2  kvm_lock_spinning (lock=0xffff88023ffe8240, want=52500) at 
arch/x86/kernel/kvm.c:786
#3  0xffffffff81037251 in __raw_callee_save_kvm_lock_spinning ()
#4  0xffff88023fdcedb0 in ?? ()
#5  0x0000000000000000 in ?? ()


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