linux-kernel - Re: [PATCH 1/2] hung_task: Show the blocker task if the task is hung on mutex

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: <20250219212708.78b93a84@gandalf.local.home>
Date: Wed, 19 Feb 2025 21:27:08 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Waiman Long <llong@...hat.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon
 <will@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Boqun Feng
 <boqun.feng@...il.com>, Joel Granados <joel.granados@...nel.org>, Anna
 Schumaker <anna.schumaker@...cle.com>, Lance Yang <ioworker0@...il.com>,
 Kent Overstreet <kent.overstreet@...ux.dev>, Yongliang Gao
 <leonylgao@...cent.com>, Tomasz Figa <tfiga@...omium.org>, Sergey
 Senozhatsky <senozhatsky@...omium.org>, linux-kernel@...r.kernel.org, Linux
 Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] hung_task: Show the blocker task if the task is
 hung on mutex

On Wed, 19 Feb 2025 21:15:08 -0500
Waiman Long <llong@...hat.com> wrote:

> Writer - setting them:
> 
>      WRITE_ONCE(lock)
>      smp_wmb()
>      WRITE_ONCE(type)
> 
> Clearing them:
> 
>      WRITE_ONCE(type, 0)
>      smp_wmb()
>      WRITE_ONCE(lock, NULL)
> 
> Reader:
> 
>      READ_ONCE(type)
> again:
>      smp_rmb()
>      READ_ONCE(lock)
>      smp_rmb()
>      if (READ_ONCE(type) != type)
>          goto again

Do you really need the READ/WRITE_ONCE() with the memory barriers? From
what I understand, the compiler can't even assume what it read is the same
after passing a memory barrier like that. So there should be no reason it
can reread the memory location after a barrier.

-- Steve