Date: Wed, 11 Mar 2015 21:14:49 -0700 From: Davide Italiano <davide@freebsd.org> To: Ryan Stone <rysto32@gmail.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, Tiwei Bie <btw@mail.ustc.edu.cn>, Mateusz Guzik <mjguzik@gmail.com>, wca@freebsd.org Subject: Re: [PATCH] Finish the task 'Convert mountlist_mtx to rwlock' Message-ID: <CACYV=-H%2BE4knx7DwaKv4qtg5xnG7bKE_E2aygYUocGtwKKqKOg@mail.gmail.com> In-Reply-To: <CAFMmRNyLJgLuk-VPSuyBCpO1bkdmyGf3s89X-An1vCCMwn=B=A@mail.gmail.com> References: <1426079434-51568-1-git-send-email-btw@mail.ustc.edu.cn> <CAFMmRNyLJgLuk-VPSuyBCpO1bkdmyGf3s89X-An1vCCMwn=B=A@mail.gmail.com>
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On Wed, Mar 11, 2015 at 9:06 PM, Ryan Stone <rysto32@gmail.com> wrote:
> On Wed, Mar 11, 2015 at 9:10 AM, Tiwei Bie <btw@mail.ustc.edu.cn> wrote:
>> Hi, Mateusz!
>>
>> I have finished the task: Convert mountlist_mtx to rwlock [1].
>
> My first comment is, are we sure that we actually want an rwlock here
> instead of an rmlock? An rmlock will offer much better performance in
> workloads that mostly only take read locks, and rmlocks do not suffer
> the priority inversion problems that rwlocks do. From the description
> on the wiki page, it sounds like an rmlock would be ideal here:
>
Snippet:
[...]
- mtx_unlock(&mountlist_mtx);
+ rw_runlock(&mountlist_lock);
mp->mnt_kern_flag |= MNTK_MWAIT;
msleep(mp, MNT_MTX(mp), PVFS | PDROP, "vfs_busy", 0);
if (flags & MBF_MNTLSTLOCK)
- mtx_lock(&mountlist_mtx);
+ rw_rlock(&mountlist_lock);
[...]
My understanding is that readers are not allowed to sleep while
holding an rmlock() so a drop-in replacement don't work in this case.
--
Davide
"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare
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