f_offset

[Jeff Roberson]

On Sun, 13 Apr 2008, Poul-Henning Kamp wrote:

> In message <20080412221654.S959 at desktop>, Jeff Roberson writes:
>
>>> The non p-prefix versions should always be serialized, because there
>>> is know way of knowing where they read/write if you don't.
>>
>> Well that's at odds with what the standard says and what others implement.
>> I think there is a clear case for serializing writes.  I don't see what
>> advantage we get from serializing reads.  The heavy cost of
>> synchronization should be justified by actual need.
>
> If you don't serialize read(2) and readv(2), how do you know where
> they read from ?

Concurrent calls to read() are inherently racy.  They will still use the 
current value of f_offset and store it while they are done.  I'm just 
suggesting we don't use an exclusive lock that is held for the duration of 
the io to protect the update to the f_offset field.  The field will still 
be updated in such a way that it is atomic.

Thanks,
Jeff

>
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
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>