linux-kernel - [QUESTION] inconsistent use of smp

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-Id: <20241204064818.2760263-1-zilin@seu.edu.cn>
Date: Wed,  4 Dec 2024 06:48:18 +0000
From: Zilin Guan <zilin@....edu.cn>
To: dhowells@...hat.com
Cc: jlayton@...nel.org,
	netfs@...ts.linux.dev,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	xujianhao01@...il.com
Subject: [QUESTION] inconsistent use of smp_mb()

Hello,

I have a question regarding the use of smp_rmb() to enforce 
memory ordering in two related functions.

In the function netfs_unbuffered_write_iter_locked() from the file 
fs/netfs/direct_write.c, smp_rmb() is explicitly used after the 
wait_on_bit() call to ensure that the error and transferred fields are 
read in the correct order following the NETFS_RREQ_IN_PROGRESS flag:

105	wait_on_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS,
106		    TASK_UNINTERRUPTIBLE);
107	smp_rmb(); /* Read error/transferred after RIP flag */
108	ret = wreq->error;
109	if (ret == 0) {
110		ret = wreq->transferred;
111		iocb->ki_pos += ret;
112	}

However, in the function netfs_end_writethrough() from the file 
fs/netfs/write_issue.c, there is no such use of smp_rmb() after 
the corresponding wait_on_bit() call, despite accessing the same filed 
of wreq->error and relying on the same NETFS_RREQ_IN_PROGRESS flag:

681	wait_on_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS, 
		    TASK_UNINTERRUPTIBLE);
682	ret = wreq->error;

My question is why does the first function require a CPU memory barrier 
smp_rmb() to enforce ordering, whereas the second function does not?

Thank you for your time and assistance!

Best Regards,
Zilin Guan