linux-kernel - Re: sys_write() racy for multi-threaded append?

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Message-ID: <20070309013405.GI6209@kvack.org>
Date:	Thu, 8 Mar 2007 20:34:05 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	"Michael K. Edwards" <medwards.linux@...il.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: sys_write() racy for multi-threaded append?

> Actually, I think it would make the kernel (negligibly) faster to bump
> f_pos before the vfs_write() call.  Unless fget_light sets fput_needed
> or the write doesn't complete cleanly, you won't have to touch the
> file table entry again after vfs_write() returns.  You can adjust

Any number of things can cause a short write to occur, and rewinding the 
file position after the fact is just as bad.  A sane app has to either 
serialise the writes itself or use a thread safe API like pwrite().

		-ben
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