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Date:      Sun, 17 Apr 2011 15:49:48 -0400 (EDT)
From:      Rick Macklem <rmacklem@uoguelph.ca>
To:        freebsd-hackers@freebsd.org
Subject:   SMP question w.r.t. reading kernel variables
Message-ID:  <397135152.167477.1303069788579.JavaMail.root@erie.cs.uoguelph.ca>
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Hi,

I should know the answer to this, but... When reading a global kernel
variable, where its modifications are protected by a mutex, is it
necessary to get the mutex lock to just read its value?

For example:
A    if ((mp->mnt_kern_flag & MNTK_UNMOUNTF) != 0)
          return (EPERM);
versus
B    MNT_ILOCK(mp);
     if ((mp->mnt_kern_flag & MNTK_UNMOUNTF) != 0) {
          MNT_IUNLOCK(mp);
          return (EPERM);
     }
     MNT_IUNLOCK(mp);

My hunch is that B is necessary if you need an up-to-date value
for the variable (mp->mnt_kern_flag in this case).

Is that correct?

Thanks in advance for help with this, rick

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