Re: Sharing a single mutex between a socket and its PCB

  > BSD/OS does:

  > tcp_usrreq(so)
  > {
  >	sb = so->so_buf;
  >	SOCKBUF_UNLOCK(sb);
  >	/* XXX What if someone modified the socket? */
  >	INP_LOCK(inp);
  >	SOCKBUF_LOCK(sb);
  >	tcp_do_usrreq();
  >	INP_UNLOCK(inp);
  > }

The XXX comment is yours.  You're thinking locally instead of globally.
SMP locking is tricky and fragile because you have to consider all the
related code together, not individual accesses to structure fields in
isolation.

Find the two actual code paths in the source where a race condition arises
due to the unlocking above.  (Then convince yourself why that race condition
doesn't really matter.)




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