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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