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

From: Hao Peng
Date: Sun Oct 09 2022 - 07:45:54 EST

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.
> 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.
> [*]