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fd code multithreaded race?
I'm looking at a KASSERT which is triggering quite rarely for me (in
terms of iterations):
panic: kernel diagnostic assertion "dt->dt_ff[i]->ff_refcnt == 0" failed: file
Upon closer examination, it seems that this can trigger while another
thread is in fd_getfile() between upping the refcount, testing for
ff_file, and fd_putfile(). Removing the KASSERT seems to restore correct
operation, but I didn't read the code far enough to see where the race
is actually handled and what stops the code from using the wrong file.
Run tests/fs/puffs/t_fuzz mountfuzz7 in a loop. A multiprocessor kernel
might produce a more reliable result, so set RUMP_NCPU unless you have
a multiprocessor host. Depending on timings and how the get/put thread
runs, you might even see the refcount as 0 in the core.
Does anyone see something wrong with the analysis? If not, I'll create
a dedidated test and file a PR.
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