linux-kernel - Re: [RFC PATCH v2] fs/namespace: use percpu_rw

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Message-ID: <877duyryhm.fsf@jogness.linutronix.de>
Date:   Mon, 20 Jul 2020 17:41:01 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH v2] fs/namespace: use percpu_rw_semaphore for writer holding

On 2020-07-19, Al Viro <viro@...iv.linux.org.uk> wrote:
>> The MNT_WRITE_HOLD flag is used to manually implement a rwsem.
>
> Could you show me where does it currently sleep?  Your version does,
> unless I'm misreading it...

You are reading it correctly. This patch introduces new possible
sleeping for__mnt_want_write() when writers for a superblock are being
held.

The RFCv1 [0] is a variant that does not introduce sleeping, but instead
reverts back to per-cpu spinlocks.

Sebastian and I are requesting comments on these two possible
solutions. Or perhaps you have an idea how to solve the potential live
lock situation.

John Ogness

[0] https://lkml.kernel.org/r/20200617104058.14902-2-john.ogness@linutronix.de