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Date: Wed, 18 Jan 2017 21:19:00 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 13/14] raw-posix: Implement image locking
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To: Max Reitz <mreitz@redhat.com>
Cc: qemu-devel@nongnu.org, "Daniel P. Berrange" <berrange@redhat.com>, Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, rjones@redhat.com

On Wed, 01/18 14:02, Max Reitz wrote:
> >> Testing whether something is locked would be easier by using F_OFD_GETLK
> >> instead of actually creating an exclusive lock and then releasing it.
> > 
> > My attempt to do this shows it doesn't work: fcntl forces the tested lock type
> > to read lock for some unknown reason, so it cannot be used to test other shared
> > lockers.
> 
> Do you have a reproducer? The attached quick and dirty test case works
> for me (compile test.c to test and test2.c to test2), producing the
> following output (when running ./test):
> 
> set rd lock: 0, Success
> get lock: 0, Success; read lock in place
> set wr lock: -1, Resource temporarily unavailable
> unset lock: 0, Resource temporarily unavailable
> ===
> unset lock: 0, Success
> get lock: 0, Success; unlocked
> set wr lock: 0, Success
> unset lock: 0, Success
> ===
> set wr lock: 0, Success
> get lock: 0, Success; write lock in place
> set wr lock: -1, Resource temporarily unavailable
> unset lock: 0, Resource temporarily unavailable

You are right, now I see why I was confused with the F_OFD_GETLK interface.

Fam

> 
> So the l_type field is correctly set after every F_OFD_GETLK query call.
> 
> Max