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From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: hch@lst.de, torvalds@linux-foundation.org,
	akpm@linux-foundation.org, James.Bottomley@hansenpartnership.com,
	guoren@kernel.org, tsbogend@alpha.franken.de,
	nickhu@andestech.com, green.hu@gmail.com, deanbo422@gmail.com,
	deller@gmx.de, ysato@users.sourceforge.jp, dalias@libc.org,
	geoff@infradead.org, paul@crapouillou.net,
	ulf.hansson@linaro.org, alexs@kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org,
	linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
	linux-sh@vger.kernel.org, linux-mmc@vger.kernel.org,
	linux-scsi@vger.kernel.org, linux-mm@kvack.org,
	linux-doc@vger.kernel.org
Subject: Re: flush_kernel_dcache_page fixes and removal
Date: Mon, 19 Jul 2021 16:42:38 +0100	[thread overview]
Message-ID: <20210719154238.GS22278@shell.armlinux.org.uk> (raw)
In-Reply-To: <20210719053851.GA16780@gondor.apana.org.au>

On Mon, Jul 19, 2021 at 01:38:51PM +0800, Herbert Xu wrote:
> Russell King Oracle <linux@armlinux.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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

WARNING: multiple messages have this Message-ID
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: hch@lst.de, torvalds@linux-foundation.org,
	akpm@linux-foundation.org, James.Bottomley@hansenpartnership.com,
	guoren@kernel.org, tsbogend@alpha.franken.de,
	nickhu@andestech.com, green.hu@gmail.com, deanbo422@gmail.com,
	deller@gmx.de, ysato@users.sourceforge.jp, dalias@libc.org,
	geoff@infradead.org, paul@crapouillou.net,
	ulf.hansson@linaro.org, alexs@kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org,
	linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
	linux-sh@vger.kernel.org, linux-mmc@vger.kernel.org,
	linux-scsi@vger.kernel.org, linux-mm@kvack.org,
	linux-doc@vger.kernel.org
Subject: Re: flush_kernel_dcache_page fixes and removal
Date: Mon, 19 Jul 2021 16:42:38 +0100	[thread overview]
Message-ID: <20210719154238.GS22278@shell.armlinux.org.uk> (raw)
In-Reply-To: <20210719053851.GA16780@gondor.apana.org.au>

On Mon, Jul 19, 2021 at 01:38:51PM +0800, Herbert Xu wrote:
> Russell King Oracle <linux@armlinux.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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2021-07-19 16:12 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-12  6:09 Christoph Hellwig
2021-07-12  6:09 ` Christoph Hellwig
2021-07-12  6:09 ` [PATCH 1/6] mmc: JZ4740: remove the flush_kernel_dcache_page call in jz4740_mmc_read_data Christoph Hellwig
2021-07-12  6:09   ` Christoph Hellwig
2021-07-19  8:15   ` Paul Cercueil
2021-07-19  8:15     ` Paul Cercueil
2021-08-04 11:12   ` Ulf Hansson
2021-08-04 11:12     ` Ulf Hansson
2021-08-04 11:12     ` Ulf Hansson
2021-07-12  6:09 ` [PATCH 2/6] mmc: mmc_spi: replace flush_kernel_dcache_page with flush_dcache_page Christoph Hellwig
2021-07-12  6:09   ` Christoph Hellwig
2021-08-04 11:12   ` Ulf Hansson
2021-08-04 11:12     ` Ulf Hansson
2021-08-04 11:12     ` Ulf Hansson
2021-07-12  6:09 ` [PATCH 3/6] ps3disk: " Christoph Hellwig
2021-07-12  6:09   ` Christoph Hellwig
2021-07-19  3:07   ` Geoff Levand
2021-07-19  3:07     ` Geoff Levand
2021-07-12  6:09 ` [PATCH 4/6] scatterlist: " Christoph Hellwig
2021-07-12  6:09   ` Christoph Hellwig
2021-07-12  6:09 ` [PATCH 5/6] aacraid: remove an unused include Christoph Hellwig
2021-07-12  6:09   ` Christoph Hellwig
2021-07-19  1:50   ` Martin K. Petersen
2021-07-19  1:50     ` Martin K. Petersen
2021-07-12  6:09 ` [PATCH 6/6] mm: remove flush_kernel_dcache_page Christoph Hellwig
2021-07-12  6:09   ` Christoph Hellwig
2021-07-12 23:56   ` Ira Weiny
2021-07-12 23:56     ` Ira Weiny
2021-07-12 19:24 ` flush_kernel_dcache_page fixes and removal Linus Torvalds
2021-07-12 19:24   ` Linus Torvalds
2021-07-12 19:24   ` Linus Torvalds
2021-07-13  5:46   ` Christoph Hellwig
2021-07-13  5:46     ` Christoph Hellwig
2021-07-13  8:46 ` Russell King (Oracle)
2021-07-13  8:46   ` Russell King (Oracle)
2021-07-13  9:11   ` Christoph Hellwig
2021-07-13  9:11     ` Christoph Hellwig
2021-07-19  5:38   ` Herbert Xu
2021-07-19  5:38     ` Herbert Xu
2021-07-19 15:42     ` Russell King (Oracle) [this message]
2021-07-19 15:42       ` Russell King (Oracle)
2021-07-14  3:13 ` Guo Ren
2021-07-14  3:13   ` Guo Ren
2021-07-14  3:13   ` Guo Ren

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