linux-kernel - [PATCH RFC v4 10/11] scsi: ufs: core: Remove unnecessary wmb() after ringing doorbell

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>] [day] [month] [year] [list]

Message-ID: <20240122-ufs-reset-ensure-effect-before-delay-v4-10-90a54c832508@redhat.com>
Date: Mon, 22 Jan 2024 11:24:06 -0600
From: Andrew Halaney <ahalaney@...hat.com>
To: Andy Gross <agross@...nel.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Manivannan Sadhasivam <mani@...nel.org>,
	"James E.J. Bottomley" <jejb@...ux.ibm.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Hannes Reinecke <hare@...e.de>,
	Janek Kotas <jank@...ence.com>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Avri Altman <avri.altman@....com>,
	Bart Van Assche <bvanassche@....org>,
	Can Guo <quic_cang@...cinc.com>
Cc: Andrew Halaney <ahalaney@...hat.com>,
	Will Deacon <will@...nel.org>,
	linux-arm-msm@...r.kernel.org,
	linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH RFC v4 10/11] scsi: ufs: core: Remove unnecessary wmb()
 after ringing doorbell

Currently, the doorbell is written to and a wmb() is used to commit it
immediately.

wmb() ensures that the write completes before following writes occur,
but completion doesn't mean that it isn't stored in a buffer somewhere.
The recommendation for ensuring this bit has taken effect on the device
is to perform a read back to force it to make it all the way to the
device. This is documented in device-io.rst and a talk by Will Deacon on
this can be seen over here:

    https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678

But, completion and taking effect aren't necessary to guarantee here.

There's already other examples of the doorbell being rung that don't do
this. The writel() of the doorbell guarantees prior writes by this
thread (to the request being setup for example) complete prior to the
ringing of the doorbell, and the following
wait_for_completion_io_timeout() doesn't require any special memory
barriers either.

With that in mind, just remove the wmb() altogether here.

Fixes: ad1a1b9cd67a ("scsi: ufs: commit descriptors before setting the doorbell")
Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/ufs/core/ufshcd.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
index 9bf490bb8eed..21f93d8e5818 100644
--- a/drivers/ufs/core/ufshcd.c
+++ b/drivers/ufs/core/ufshcd.c
@@ -7047,10 +7047,7 @@ static int __ufshcd_issue_tm_cmd(struct ufs_hba *hba,

 	/* send command to the controller */
 	__set_bit(task_tag, &hba->outstanding_tasks);
-
 	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TASK_REQ_DOOR_BELL);
-	/* Make sure that doorbell is committed immediately */
-	wmb();

 	spin_unlock_irqrestore(host->host_lock, flags);

-- 
2.43.0