Date: Wed, 11 Mar 2015 21:44:39 -0700 From: Mark Johnston <markj@FreeBSD.org> To: Davide Italiano <davide@freebsd.org> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, Tiwei Bie <btw@mail.ustc.edu.cn>, Mateusz Guzik <mjguzik@gmail.com>, Ryan Stone <rysto32@gmail.com>, wca@freebsd.org Subject: Re: [PATCH] Finish the task 'Convert mountlist_mtx to rwlock' Message-ID: <20150312044439.GB11120@raichu> In-Reply-To: <CACYV=-H%2BE4knx7DwaKv4qtg5xnG7bKE_E2aygYUocGtwKKqKOg@mail.gmail.com> References: <1426079434-51568-1-git-send-email-btw@mail.ustc.edu.cn> <CAFMmRNyLJgLuk-VPSuyBCpO1bkdmyGf3s89X-An1vCCMwn=B=A@mail.gmail.com> <CACYV=-H%2BE4knx7DwaKv4qtg5xnG7bKE_E2aygYUocGtwKKqKOg@mail.gmail.com>
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On Wed, Mar 11, 2015 at 09:14:49PM -0700, Davide Italiano wrote: > 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. That's true of an rwlock as well though. And the mountlist lock is dropped around the msleep call in this snippet.
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