linux-kernel - Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put

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Message-ID: <mvmikxvonjh.fsf@suse.de>
Date: Wed, 26 Jun 2024 15:12:50 +0200
From: Andreas Schwab <schwab@...e.de>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,  Palmer Dabbelt
 <palmer@...belt.com>,  Albert Ou <aou@...s.berkeley.edu>,
  linux-riscv@...ts.infradead.org,  linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of
 __put_user

On Jun 25 2024, Jisheng Zhang wrote:

> I believe the output constraints "=m" is not necessary, because
> the instruction itself is "write", we don't need the compiler
> to "write" for us.

No, this is backwards.  Being an output operand means that the *asm* is
writing to it, and the compiler can read the value from there afterwards
(and the previous value is dead before the asm).

-- 
Andreas Schwab, SUSE Labs, schwab@...e.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."