linux-kernel - Re: SMP read() stopping at memory page boundaries

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Message-Id: <1182354493.3768.80.camel@hurina>
Date:	Wed, 20 Jun 2007 18:48:13 +0300
From:	Timo Sirainen <tss@....fi>
To:	linux-kernel@...r.kernel.org
Subject: Re: SMP read() stopping at memory page boundaries

On Wed, 2007-06-20 at 17:52 +0300, Timo Sirainen wrote:
> Sometimes read() returns only 4096 bytes. I'm locking the file, so I
> don't think this should ever happen, right?

Sorry, the problem was with file truncating so there's no bug with
locking. I didn't notice it first because it happened to work with
FreeBSD and Solaris.

Without locking there's a problem that I'd want to avoid, but I guess
it's not necessarily a bug (although it doesn't seem to happen with
FreeBSD or Solaris):

Process 1:

 - create "foo2"
 - rename() to "foo"
 - lock
 - write(4096 + 16 bytes)
 - [fsync() here doesn't change anything]
 - pwrite("1111" to offset=4096-4)
 - unlock

Process 2:

 - open("foo")
 - read(8192 bytes)

read() sometimes returns 4096 bytes but with the "1111" already included
in the data. Is there a way to avoid this without locking the file while
reading? The "1111" tries to act as a kind of a lock.

Again attached a test program, which should really do what I
intended. :)

View attachment "concurrency.c" of type "text/x-csrc" (1718 bytes)

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