Bug 53647

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Description of problem:
One process is writing text lines to a file.  Another process is reading
those lines as they are written.  Using 2.2.x kernels and/or 2.4.x
uniprocessor kernels, the reading program always reads complete lines. 
Using a 2.4.x SMP kernel, the reading process sometimes reads partial
lines.
This happens regardless of the i/o type (stream or binary) used., and
mandatory file locking does not appear to help.

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. The writing program loops:  opens the file for append, writes a single
complete line of text, and then closes the file. The reading program: opens
the file for input, then loops reading from the file.   The lines are a
fixed size (80 chars) terminated with a newline.
2. A modified version of the writing program tries to use mandatory file
locking to protect the write operation, without success.
3.
	

Actual Results:  Under 2.2 kernels, or 2.4.x uniprocessor, the reading
program always reads a complete line.

Under an SMP 2.4.x kernel, the reading program usually reads a complete
line, but occasionally (depending on the speed of both programs) reads only
a partial line.


Expected Results:  The expected behavior without mandatory file locking
does not appear to be documented, but it should be consistent between uni-
and multi-processor kernels.

Using mandatory file locking in the writing program should insure that the
reading program always reads complete lines.

Additional info:

This has been tried with the  version 2.4.2-2 kernel on the retail 7.1 CDs,
and also with the updated 2.4.3-12 kernel.

Test programs for both the reading and writing side are attached (simread.c
and simtst.c).  In actual use, the reading program is a large commercial
application ; we have no access to the sources.  The writing program was
written locally; we have the sources and can make any useful changes to it.