Messages in this thread |  | | From | Linus Torvalds <> | Date | Sat, 27 Jul 2019 09:41:25 -0700 | Subject | Re: [PATCH v2 1/2] pidfd: add P_PIDFD to waitid() |
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On Sat, Jul 27, 2019 at 9:28 AM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > Something like > > struct pid *fd_to_pid(unsigned int fd) > { > struct fd f; > struct pid *pid; ...
I forgot to put my usual disclaimer about TOTALLY UNTESTED GARBAGE in that email. I want to make that part clear: that code snippet was meant as a rough guide of direction, not as a "this works".
Hopefully that was clear.
Also note again that one of the reasons I would prefer that "fd_to_pid()" interface is that you _can_ do it cleverly with RCU lookup, but that requires a lot of care.
In particular, I think all of our _existing_ "proc_pid(file_inode(file))" users are done while you actually hold a reference to "struct file *", so they don't have to worry about races with another thread doing the final ->release(). So the "clever" thing is possible, but might need a _lot_ of care to make sure the 'struct pid *' associated with the file still exists.
The example code sequence was not doing the clever thing, obviously. So it was untested _and_ simple-stupid. But it has the interface that I'd prefer.
Linus
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