[ECOS] Coarse Grained Locking in Fileio package

[Jeffrey R. Szczepanski]

I have noticed a "problem" in the eCos fileio package (packages/io/fileio) 
relative to what amounts to rather "coarse grained" mutex protection during 
a file close() operation.

Background: When the last open'ed file descriptor on a particular file 
object is closed via close() implementation in the file
io/fileio/src/io.cxx, a call to cyg_fd_free() is required to deallocate the 
file descriptor and the file object. Since in this case, this is the last 
reference to the file object, the file system's fo_close() method is also 
called. This entire operation here is undertaken in the context of holding 
the system wide file descriptor lock named "fdlock". See 
io/fileio/src/fd.cxx implementation of cyg_fd_free().

The side of effect of this is while any file system's fo_close() operation 
is taking place ANY file descriptor operations (system wide) will be blocked 
waiting for the pending close to complete....no matter what device or 
filesystem the I/O is associated with. Since I/O device interactions are 
likely required during say a disk based file operation, it is possibly 
and/or likely that all I/O operations in the entire system that work through 
file descriptors will blocked during this time period.

I think this operation needs a much more finer grained mutex locking 
strategy.

My initial thought is that file object's reference count (f_ucount), in 
principle, should be protected by the file object's own mutex, not by the 
system wide "fdlock" mutex. This will allow the file-system's fo_close() 
method to be invoked w/o still needing to hold the fdlock mutex, thereby 
allowing all other file descriptors to operate concurrently with the close 
operation.

I believe the set of changes needed would be isolated to the 
io/fileio/src/fd.cxx module.

Thoughts/comments before I make an attempt to make the necessary changes to 
the file!?

Thanks in advance,
Jeff
jrs at inscitek dot com 


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