linux-kernel - Re: flush_kernel_dcache

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Message-ID: <20210719154238.GS22278@shell.armlinux.org.uk>
Date:   Mon, 19 Jul 2021 16:42:38 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     hch@....de, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, James.Bottomley@...senpartnership.com,
        guoren@...nel.org, tsbogend@...ha.franken.de, nickhu@...estech.com,
        green.hu@...il.com, deanbo422@...il.com, deller@....de,
        ysato@...rs.sourceforge.jp, dalias@...c.org, geoff@...radead.org,
        paul@...pouillou.net, ulf.hansson@...aro.org, alexs@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-csky@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org
Subject: Re: flush_kernel_dcache_page fixes and removal

On Mon, Jul 19, 2021 at 01:38:51PM +0800, Herbert Xu wrote:
> Russell King Oracle <linux@...linux.org.uk> wrote:
> >
> > I think you need to be careful - I seem to have a recollection that the
> > reason we ended up with flush_kernel_dcache_page() was the need to avoid
> > the taking of the mmap lock for 32-bit ARM VIVT based CPUs in
> > flush_dcache_page(). 32-bit ARM flush_dcache_page() can block.
> > 
> > If you're sure that all these changes you're making do not end up
> > calling flush_dcache_page() from a path where we are atomic, then fine.
> 
> The Crypto API has been calling flush_dcache_page from softirq
> context since before the advent of git (see crypto/scatterwalk.c
> from the initial import).  So if 32-bit ARM blocks on it then this
> has been broken for almost 20 years.

I think what's confusing me is the naming of flush_dcache_mmap_lock().
The mmap lock is a read-write semaphore (see linux/mmap-lock.h), and
is even called "mmap_lock" in mm_struct, but this has nothing to do
with flush_dcache_mmap_lock().

So no, flush_dcache_mmap_lock() doesn't block as I first thought, and
therefore flush_dcache_page() doesn't block either.

Sorry for the noise.

However, I now seem to remember some discussion in the past when I was
trying to get people to use flush_dcache_page() to solve the coherency
problems when block drivers were doing PIO to page cache pages. I seem
to remember there being objections to it, which is one of the reasons
we ended up with a lighter weight flush_kernel_dcache_page(). But
shrug, dim and distant memories.

-- 
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