linux-kernel - Re: [PATCH] block: Annotate a racy read in blk_do_io

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Date: Fri, 10 May 2024 13:30:03 -0700
From: Bart Van Assche <bvanassche@....org>
To: paulmck@...nel.org
Cc: Breno Leitao <leitao@...ian.org>, Jens Axboe <axboe@...nel.dk>,
 "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: Annotate a racy read in blk_do_io_stat()

On 5/10/24 10:08 AM, Paul E. McKenney wrote:
> To see that, consider a variable that is supposed to be accessed only
> under a lock (aside from the debugging/statistical access).  Under RCU's
> KCSAN rules, marking those debugging/statistical accesses with READ_ONCE()
> would require all the updates to be marked with WRITE_ONCE().  Which would
> prevent KCSAN from noticing a buggy lockless WRITE_ONCE() update of
> that variable.
> 
> In contrast, if we use data_race() for the debugging/statistical accesses
> and leave the normal lock-protected accesses unmarked (as normal
> C-language accesses), then KCSAN will complain about buggy lockless
> accesses, even if they are marked with READ_ONCE() or WRITE_ONCE().
> 
> Does that help, or am I missing your point?

Thanks, that's very helpful. Has it been considered to add this
explanation as a comment above the data_race() macro definition?
There may be other kernel developers who are wondering about when
to use data_race() and when to use READ_ONCE().

Thanks,

Bart.