linux-kernel - Re: [PATCH next v4 02/15] mtd: mtdoops: synchronize kmsg

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Message-ID: <20210311105055.4a0e864d@xps13>
Date:   Thu, 11 Mar 2021 10:50:55 +0100
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd@...ts.infradead.org
Subject: Re: [PATCH next v4 02/15] mtd: mtdoops: synchronize kmsg_dumper

Hello,

John Ogness <john.ogness@...utronix.de> wrote on Wed,  3 Mar 2021
11:15:15 +0100:

> The kmsg_dumper can be called from any context and CPU, possibly
> from multiple CPUs simultaneously. Since the writing of the buffer
> can occur from a later scheduled work queue, the oops buffer must
> be protected against simultaneous dumping.
> 
> Use an atomic bit to mark when the buffer is protected. Release the
> protection in between setting the buffer and the actual writing in
> order for a possible panic (immediate write) to be written during
> the scheduling of a previous oops (delayed write).
> 
> An atomic bit (rather than a spinlock) was chosen so that no
> scheduling or preemption side-effects would be introduced. The MTD
> kmsg_dumper may dump directly or it may be delayed (via scheduled
> work). Depending on the context, different MTD callbacks are used.
> For example, mtd_write() expects to be called in a non-atomic
> context and may take a mutex.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Petr Mladek <pmladek@...e.com>

Reviewed-by: Miquel Raynal <miquel.raynal@...tlin.com>

Thanks,
Miquèl