linux-kernel - RE: [PATCH 3/8] scsi: ufshpb: Add region's reads counter

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Message-ID: <BL0PR04MB6564C0EB1584AE599A13E120FCB79@BL0PR04MB6564.namprd04.prod.outlook.com>
Date:   Sun, 31 Jan 2021 07:25:37 +0000
From:   Avri Altman <Avri.Altman@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bart Van Assche <bvanassche@....org>,
        yongmyung lee <ymhungry.lee@...sung.com>,
        Daejun Park <daejun7.park@...sung.com>,
        "alim.akhtar@...sung.com" <alim.akhtar@...sung.com>,
        "asutoshd@...eaurora.org" <asutoshd@...eaurora.org>,
        Zang Leigang <zangleigang@...ilicon.com>,
        Avi Shchislowski <Avi.Shchislowski@....com>,
        Bean Huo <beanhuo@...ron.com>,
        "cang@...eaurora.org" <cang@...eaurora.org>,
        "stanley.chu@...iatek.com" <stanley.chu@...iatek.com>
Subject: RE: [PATCH 3/8] scsi: ufshpb: Add region's reads counter

> >
> > +     if (ufshpb_mode == HPB_HOST_CONTROL)
> > +             reads = atomic64_inc_return(&rgn->reads);
> > +
> >       if (!ufshpb_is_support_chunk(transfer_len))
> >               return;
> >
> > +     if (ufshpb_mode == HPB_HOST_CONTROL) {
> > +             /*
> > +              * in host control mode, reads are the main source for
> > +              * activation trials.
> > +              */
> > +             if (reads == ACTIVATION_THRSHLD) {
Oops - this is a bug...

> > +
> > +     /* region reads - for host mode */
> > +     atomic64_t reads;
> 
> Why do you need an atomic variable for this?  What are you trying to
> "protect" here by flushing the cpus all the time?  What protects this
> variable from changing right after you have read from it (hint, you do
> that above...)
> 
> atomics are not race-free, use a real lock if you need that.
We are on the data path here - this is called from queuecommand.
The "reads" counter is being symmetrically read and written,
so adding a spin lock here might become a source for contention.

Also I am not worried about the exact value of this counter, except of one place - 
See above.  Will fix it.

Thanks,
Avri