linux-kernel - Re: [PATCH next v3 6/8] printk: ringbuffer: add finalization/extension support

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Message-ID: <878sdvq8kd.fsf@jogness.linutronix.de>
Date:   Mon, 31 Aug 2020 15:00:58 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v3 6/8] printk: ringbuffer: add finalization/extension support

This critical piece was missing from patch 6...


>From 0b745d507f0c38e6d1612ed9468aa52845ca025b Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@...utronix.de>
Date: Mon, 31 Aug 2020 14:45:40 +0206
Subject: [PATCH] printk: ringbuffer: allow reading consistent descriptors

desc_read() will fail to read if a descriptor is in the desc_reserved
queried state because such data would be inconsistent. However, since
("printk: ringbuffer: add finalization/extension support") the
desc_reserved state can have the DESC_COMMIT_MASK flag set, in which
case it _is_ consistent. And indeed, desc_reopen_last() is expecting
a read in this case.

Allow desc_read() to read desc_reserved descriptors if the
DESC_COMMIT_MASK flag is set.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
Reported-by: Andy Lavr <andy.lavr@...il.com>
Fixes: ("printk: ringbuffer: add finalization/extension support")
---
 kernel/printk/printk_ringbuffer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 0731d5e2dddd..6ba7d3fc96f1 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -446,8 +446,10 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 	/* Check the descriptor state. */
 	state_val = atomic_long_read(state_var); /* LMM(desc_read:A) */
 	d_state = get_desc_state(id, state_val);
-	if (d_state != desc_committed && d_state != desc_reusable)
+	if (d_state == desc_miss ||
+	    (d_state == desc_reserved && !(state_val & DESC_COMMIT_MASK))) {
 		return d_state;
+	}
 
 	/*
 	 * Guarantee the state is loaded before copying the descriptor
-- 
2.20.1