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While writing a driver, I came across a issue mentioned below.

Given a multithreaded application accessing the same device file through same FD. Consider that between the calls to OPEN and RELEASE, there are some resources (say mutex) held mutually by the thread-group. These resources are used during the READ/WRITE calls, and then eventually given up or destroyed during RELEASE.

If there is one thread accessing the resource during READ/WRITE and another thread simultaneously invokes the RELEASE by calling close, how is it assured by the VFS that the RELEASE is not called until there is at least one thread in the READ, WRITE, or like. What mechanism is handling this protection?

user31986
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    1. Why don't you try it out? 2. Guess why it's called `release` and not `close`. – CL. Sep 25 '13 at 07:03
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    Yes, I tried that. It does not close the file until no one is reading/writing. But I am not sure what mechanism is handling that protection. – user31986 Sep 26 '13 at 13:12

1 Answers1

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The kernel layer above the device drivers keeps track of how many references to an open file exist and does not call the release function until all of those references have been closed. This is somewhat documented in LDD3: http://tjworld.net/books/ldd3/#TheReleaseMethod

Benjamin Leinweber
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