Re: [PATCH] kvm: x86: keep srcu writer side operation mutually exclusive

[Hao Peng]

On Sat, Oct 8, 2022 at 1:12 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Sat, Oct 08, 2022, Hao Peng wrote:
> > From: Peng Hao <flyingpeng@xxxxxxxxxxx>
> >
> > Synchronization operations on the writer side of SRCU should be
> > invoked within the mutex.
>
> Why?  Synchronizing SRCU is necessary only to ensure that all previous readers go
> away before the old filter is freed.  There's no need to serialize synchronization
> between writers.  The mutex ensures each writer operates on the "new" filter that's
> set by the previous writer, i.e. there's no danger of a double-free.  And the next
> writer will wait for readers to _its_ "new" filter.
>
Array srcu_lock_count/srcu_unlock_count[] in srcu_data, which is used
alternately to determine
which readers need to wait to get out of the critical area. If  two
synchronize_srcu are initiated concurrently,
there may be a problem with the judgment of gp. But if it is confirmed
that there will be no writer concurrency,
it is not necessary to ensure that synchronize_srcu is executed within
the scope of the mutex lock.
Thanks.
> I think it's a moot point though, as this is a subset of patch I posted[*] to fix
> other issues with the PMU event filter.
>
> [*] https://lore.kernel.org/all/20220923001355.3741194-2-seanjc@xxxxxxxxxx