Re: Opening of blocking FIFOs not reliable?

[Teemu Koponen]

On Aug 14, 2005, at 3:15, Manfred Spraul wrote:

> > Opening a FIFO for WR_ONLY should release a previously blocked 
> > RD_ONLY open. I suspect this is not guaranteed on a heavily loaded 
> > Linux box.
> Do you have a test case?
> IIRC we had that bug, and it was fixed by adding PIPE_WCOUNTER:
> PIPE_WRITERS counts the number of writers. This one is decreased 
> during close(). PIPE_WCOUNTER counts how often a writer was seen. It's 
> never decreased. Readers that wait for a writer wait until 
> PIPE_WCOUNTER changes, they do not look at PIPE_WRITERS.

Ah, I missed the semantics of WCOUNTER.  True, writer's open should 
always release the reader' s open (as Alan pointed out).

I do have a test application, which experienced dead-locked readers' 
opens before I artificially delayed writers' closes. The application 
runs on PlanetLab, which uses VServer, though. I'll try to construct 
next a minimal test case based on the application and reproduce the 
behavior on a vanilla box...

Teemu

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