linux-kernel - Re: [PATCH] zram: Replace bit spinlocks with spinlock_t for PREEMPT

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Message-ID: <20230324040750.GE3271889@google.com>
Date:   Fri, 24 Mar 2023 13:07:50 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] zram: Replace bit spinlocks with spinlock_t for
 PREEMPT_RT.

On (23/03/23 17:18), Sebastian Andrzej Siewior wrote:
> From: Mike Galbraith <umgwanakikbuti@...il.com>
> 
> The bit spinlock disables preemption. The spinlock_t lock becomes a sleeping
> lock on PREEMPT_RT and it can not be acquired in this context. In this locked
> section, zs_free() acquires a zs_pool::lock, and there is access to
> zram::wb_limit_lock.
> 
> Use a spinlock_t on PREEMPT_RT for locking and set/ clear ZRAM_LOCK bit after
> the lock has been acquired/ dropped.
> 
> Signed-off-by: Mike Galbraith <umgwanakikbuti@...il.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Link: https://lkml.kernel.org/r/YqIbMuHCPiQk+Ac2@linutronix.de
> ---
> 
> I'm simply forwarding Mike's patch here. The other alternative is to let
> the driver depend on !PREEMPT_RT. I can't tell likely it is that this
> driver is used. Mike most likely stumbled upon it while running LTP.

Yeah, I'm curious if anyone uses zram in preempt-rt systems. I don't
mind this patch but would be nice to add new code when it solves some
real problems. Maybe `depend on !PREEMPT_RT` can be a better option.