linux-kernel - Re: [PATCH v2] blk-cgroup: Replace u64_sync with blkg_stat

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

Message-ID: <ZpF4Pfu83W9Wp_R9@slm.duckdns.org>
Date: Fri, 12 Jul 2024 08:38:53 -1000
From: "tj@...nel.org" <tj@...nel.org>
To: Boy Wu (吳勃誼) <Boy.Wu@...iatek.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"boris@....io" <boris@....io>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	Iverlin Wang (王苳霖) <Iverlin.Wang@...iatek.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v2] blk-cgroup: Replace u64_sync with blkg_stat_lock for
 stats update

Hello, Boy.

On Fri, Jul 12, 2024 at 01:39:51AM +0000, Boy Wu (吳勃誼) wrote:
...
> I agree, but for multiple updaters, we not only need a spin lock but
> also need u64_sync for 32bit SMP systems because u64_stats_fetch is not
> protected by the spin lock blkg_stat_lock. If removing u64 sync, then
> one CPU fetches data while another CPU is updating, may get a 64 bits
> data with only 32 bits updated, while the other 32 bits are not updated
> yet. We can see that blkcg_iostats_update is protected by both u64_sync
> and the spin lock blkg_stat_lock in __blkcg_rstat_flush.
> Thus, I think we should keep the u64_sync and just add the spin
> lock blkg_stat_lock, not replace u64_sync with the spin lock.

I don't get it. The only reader of blkg->iostat is blkcg_print_one_stat().
It can just grab the same spin lock that the updaters use, right? Why do we
also need u64_sync for blkg->iostat?

Thanks.

-- 
tejun