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Subject: Re: svn commit: r356120 - head/sys/security/mac
To: Ronald Klop <ronald-lists@klop.ws>, src-committers@freebsd.org,
 svn-src-all@freebsd.org, svn-src-head@freebsd.org,
 Mateusz Guzik <mjg@freebsd.org>
References: <201912271123.xBRBNWow013539@repo.freebsd.org>
 <op.0dgfqujxkndu52@sjakie>
From: Hans Petter Selasky <hps@selasky.org>
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Date: Fri, 27 Dec 2019 11:50:02 +0100
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On 2019-12-27 12:33, Ronald Klop wrote:
> Wow, this looks like a winner.
> 
> Ronald.

Is this a spin-off from epoch methodology?

It is possible to use epoch as a part of read mostly locking too.

The basic idea here is to have a variable/refcount which when set, 
fallback to a regular mutex during read-locking. This is all 
synchronized under epoch.

Pseudo code:

read_lock() {
epoch_enter();
do_lock = writers != 0;
if (do_lock)
   mtx_lock();
}

read_unlock() {
if (do_lock)
  mtx_unlock();
epoch_exit();
}


write_lock() {
writers++;
epoch_wait();
mtx_lock();
}

write_unlock() {
mtx_unlock();
writers--;
epoch_wait();
}

--HPS