linux-kernel - [RFC PATCH 0/1] fs: remove retry loop

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Date:   Wed, 17 Jun 2020 12:46:57 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Alexander Viro <viro@...iv.linux.org.uk>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 0/1] fs: remove retry loop

Hello,

This patch removes the last retry loop in VFS. It is partially
reverting

   commit d3ef3d7351cc ("fs: mnt_want_write speedup")

by re-introducing per-cpu spinlocks for each mount. The patch
includes benchmark results in the diffstat section to show that the
previous optimization work is not undone.

I would have liked to use a percpu_rw_semaphore per mount instead
of the many spinlocks. However, percpu_rw_semaphore can sleep, which
is a problem for sb_prepare_remount_readonly() since it needs to
take the spinlock in @mount_lock in order to iterate @sb->s_mounts.
Perhaps using a mutex to sychronize @sb->s_mounts is an option. I am
not sure. That is why this is an RFC.

I am suggesting this partial revert because it removes the retry
loop and does not show any obvious negative benchmark effects.

John Ogness (1):
  fs/namespace.c: use spinlock instead of busy loop

 fs/mount.h     |   7 +++
 fs/namespace.c | 118 +++++++++++++++++++++++++++++++++----------------
 2 files changed, 86 insertions(+), 39 deletions(-)

-- 
2.20.1