Re: [Qemu-devel] [PATCH v2 08/12] cpus: always call seqlock_write in cpu_update_icount

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/09/2018 14:53, Richard Henderson wrote:
>>  
>> -#ifndef CONFIG_ATOMIC64
>>      seqlock_write_lock(&timers_state.vm_clock_seqlock,
>>                         &timers_state.vm_clock_lock);
>> -#endif
>>      atomic_set__nocheck(&timers_state.qemu_icount,
>>                          timers_state.qemu_icount + executed);
>> -#ifndef CONFIG_ATOMIC64
>>      seqlock_write_unlock(&timers_state.vm_clock_seqlock,
>>                           &timers_state.vm_clock_lock);
>> -#endif
> I don't understand this.  Surely you either want atomic64_set,
> or an actual atomic64_add, but no extra lock when CONFIG_ATOMIC64.

Even though writes of qemu_icount can safely race with reads in
qemu_icount_raw, qemu_icount is also read by icount_adjust, which runs
in the I/O thread.  Therefore, writes do need protection of the
vm_clock_lock; for simplicity Emilio's patch protects it with both
seqlock+spinlock, which we already do for hosts that lack 64-bit atomics.

The bug actually predated the introduction of vm_clock_lock;
cpu_update_icount would have needed the BQL before the spinlock was
introduced.

The above could have been a commit message, by the way. :)

Paolo