linux-kernel - Re: [PATCH v2] KVM: selftests: Fix the dirty_log

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Date: Thu, 1 Feb 2024 09:32:41 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Shaoqin Huang <shahuang@...hat.com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev, 
	Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2] KVM: selftests: Fix the dirty_log_test semaphore imbalance

On Thu, Feb 01, 2024, Shaoqin Huang wrote:
> > >   	/*
> > >   	 * We reserve page table for 2 times of extra dirty mem which
> > >   	 * will definitely cover the original (1G+) test range.  Here
> > > @@ -825,6 +832,13 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> > >   		sync_global_to_guest(vm, iteration);
> > >   	}
> > > +	/*
> > > +	 *
> > > +	 * Before we set the host_quit, let the vcpu has time to run, to make
> > > +	 * sure we consume the sem_vcpu_stop and the vcpu consume the
> > > +	 * sem_vcpu_cont, to keep the semaphore balance.
> > > +	 */
> > > +	usleep(p->interval * 1000);
> > 
> > Please no.  "Wait for a while" is never a complete solution for fixing races.
> > In rare cases, adding a delay might be the only sane workaround, but I doubt that's
> > the case here.
> 
> If that's the case. I guess I should keep the current solution. Except you
> have any better solution, please let me know.

Unfortunately I don't have a better solution, and I don't have cycles to stare
at this deeply to figure out what how to make the synchronization rock solid.
Sorry :-/