CVS commit: src/sys/kern

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Thu Feb 23 02:58:41 UTC 2023

Modified Files:
        src/sys/kern: kern_descrip.c

Log Message:
kern_descrip.c: Use atomic_store_relaxed/release for ff->ff_file.

1. atomic_store_relaxed in fd_close avoids the appearance of race in
   sanitizers (minor bug).

2. atomic_store_release in fd_affix is necessary because the lock
   activity was not, in fact, enough to guarantee ordering (real bug
   some architectures like aarch64).

   The premise appears to have been that the mutex_enter/exit earlier
   in fd_affix is enough to guarantee that initialization of fp (A)
   happens before use of fp by a user once fp is published (B):

        fp->f_... = ...;                // A

        /* fd_affix */
        mutex_enter(&fp->f_lock);
        fp->f_count++;
        mutex_exit(&fp->f_lock);
        ...
        ff->ff_file = fp;               // B

   But actually mutex_enter/exit allow the following reordering by
   the CPU:

        mutex_enter(&fp->f_lock);
        ff->ff_file = fp;               // B
        fp->f_count++;
        fp->f_... = ...;                // A
        mutex_exit(&fp->f_lock);

   The only constraints they imply are:

        1. fp->f_count++ and B cannot precede mutex_enter
        2. mutex_exit cannot precede A and fp->f_count++

   They imply no constraint on the relative ordering of A, B, and
   fp->f_count++ amongst each other, however.

   This affects any architecture that has a native load-acquire or
   store-release operation in mutex_enter/exit, like aarch64, instead
   of explicit load-before-load/store and load/store-before-store
   barrier.

No need for atomic_store_* in fd_copy or fd_free because we have
exclusive access to ff as is.

XXX pullup-9
XXX pullup-10


To generate a diff of this commit:
cvs rdiff -u -r1.252 -r1.253 src/sys/kern/kern_descrip.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.