linux-kernel - Re: [PATCH v2 RESEND 7/7] swiotlb: per-device flag if there are dynamically allocated buffers

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Message-ID: <20230515120054.0115a4eb@meshulam.tesarici.cz>
Date:   Mon, 15 May 2023 12:00:54 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Petr Tesarik <petrtesarik@...weicloud.com>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Kim Phillips <kim.phillips@....com>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH v2 RESEND 7/7] swiotlb: per-device flag if there are
 dynamically allocated buffers

On Mon, 15 May 2023 10:48:47 +0200
Petr Tesařík <petr@...arici.cz> wrote:

> Hi Catalin,
> 
> On Sun, 14 May 2023 19:54:27 +0100
> Catalin Marinas <catalin.marinas@....com> wrote:
>[...]
> > Now, thinking about the list_head access and the flag ordering, since it
> > doesn't matter, you might as well not bother with the flag at all and
> > rely on list_add() and list_empty() ordering vs the hypothetical 'blah'
> > access. Both of these use READ/WRITE_ONCE() for setting
> > dma_io_tlb_dyn_slots.next. You only need an smp_wmb() after the
> > list_add() and an smp_rmb() before a list_empty() check in
                      ^^^^^^^^^
Got it, finally. Well, that's exactly something I don't want to do.
For example, on arm64 (seeing your email address), smp_rmb() translates
to a "dsb ld" instruction. I would expect that this is more expensive
than a "ldar", generated by smp_load_acquire().

I mean, for devices that do not need swiotlb, the flag never changes
from zero, so reading it must be as cheap as possible.

Petr T

> > is_swiotlb_buffer(), no dma_iotlb_have_dyn variable.  
> 
> Wait, let me check that I understand you right. Do you suggest that I
> convert dma_io_tlb_dyn_slots to a lockless list and get rid of the
> spinlock?
> 
> I'm sure it can be done for list_add() and list_del(). I'll have
> to think about list_move().

Hm, even the documentation of llist_empty() says that it is "not
guaranteed to be accurate or up to date". If it could be, I'm quite
sure the authors would have gladly implemented it as such.

Petr T