From: Al Viro <viro@zeniv.linux.org.uk>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Karel Zak <kzak@redhat.com>, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2] proc/mounts: add cursor
Date: Thu, 9 Apr 2020 17:50:48 +0100 [thread overview]
Message-ID: <20200409165048.GE23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200409141619.GF28467@miu.piliscsaba.redhat.com>
On Thu, Apr 09, 2020 at 04:16:19PM +0200, Miklos Szeredi wrote:
> Solve this by adding a cursor entry for each open instance. Taking the
> global namespace_sem for write seems excessive, since we are only dealing
> with a per-namespace list. Instead add a per-namespace spinlock and use
> that together with namespace_sem taken for read to protect against
> concurrent modification of the mount list. This may reduce parallelism of
> is_local_mountpoint(), but it's hardly a big contention point. We could
> also use RCU freeing of cursors to make traversal not need additional
> locks, if that turns out to be neceesary.
Umm... That can do more than reduction of parallelism - longer lists take
longer to scan and moving cursors dirties cachelines in a bunch of struct
mount instances. And I'm not convinced that your locking in m_next() is
correct.
What's to stop umount_tree() from removing the next entry from the list
just as your m_next() tries to move the cursor? I don't see any common
locks for those two...
next prev parent reply other threads:[~2020-04-09 16:50 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-09 14:16 [PATCH v2] proc/mounts: add cursor Miklos Szeredi
2020-04-09 16:22 ` Aurélien Aptel
2020-04-09 16:50 ` Al Viro [this message]
2020-04-09 16:54 ` Al Viro
2020-04-09 18:30 ` Al Viro
2020-04-09 19:36 ` Miklos Szeredi
2020-04-09 18:45 ` Matthew Wilcox
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