linux-kernel - Re: [PATCH] ring-buffer: use READ_ONCE() to read cpu_buffer->commit

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <tencent_746C490722E89EE23629C2988896C8D02F05@qq.com>
Date: Thu, 29 Feb 2024 20:32:26 +0800
From: linke <lilinke99@...com>
To: rostedt@...dmis.org
Cc: lilinke99@...com,
 linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org,
 mathieu.desnoyers@...icios.com,
 mhiramat@...nel.org
Subject: Re: [PATCH] ring-buffer: use READ_ONCE() to read
 cpu_buffer->commit_page in concurrent environment

Hi Steven, sorry for the late reply.

> 
> Now the reason for the above READ_ONCE() is because the variables *are*
> going to be used again. We do *not* want the compiler to play any games
> with that.
> 

I don't think it is because the variables are going to be used again. 
Compiler optimizations barely do bad things in single thread programs. It
is because cpu_buffer->commit_page may change concurrently and should be
accessed atomically.

	/* Make sure commit page didn't change */
	curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
	curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);

	/* If the commit page changed, then there's more data */
	if (curr_commit_page != commit_page ||
	    curr_commit_ts != commit_ts)
		return 0;

This code read cpu_buffer->commit_page and time_stamp again to check
whether commit page changed. It shows that cpu_buffer->commit_page and 
time_stamp may be changed by other threads.

        commit_page = cpu_buffer->commit_page;
        commit_ts = commit_page->page->time_stamp;

So the commit_page and time_stamp above is read while other threads may
change it. I think it is a data race if it is not atomic. Thus it is 
necessary to use READ_ONCE() here.