Re: VFS locking: f_pos thread-safe ?
From: Andrew Morton
Date: Fri Feb 06 2004 - 02:54:13 EST
Werner Almesberger <wa@xxxxxxxxxxxxxxx> wrote:
> "[...] read( ) [...] shall be atomic with respect to each other
> in the effects specified in IEEE Std. 1003.1-200x when they
> operate on regular files. If two threads each call one of these
> functions, each call shall either see all of the specified
> effects of the other call, or none of them."
Whichever thread finishes its read last gets to update f_pos.
I'm struggling a bit to understand what they're calling for there. If
thread A enters a read and then shortly afterwards thread B enters the
read, does thread B see an f_pos which starts out at the beginning of A's
read, or the end of it?
Similar questions apply as the threads exit their read()s.
Either way, there's no way in which we should serialise concurrent readers.
That would really suck for sensible apps which are using pread64().
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