Dynamic reads without locking.

[Harti Brandt]

On Thu, 9 Oct 2003, Jeffrey Hsu wrote:

JH>  > I'm wondering...
JH>  > Jeffrey Hsu was talking about this at BSDCon03.
JH>  > There is no need to lock data when we just made simple read, for example:
JH>  >
JH>  >     mtx_lock(&foo_mtx);
JH>  >     foo = 5;
JH>  >     mtx_unlock(&foo_mtx);
JH>  > but only:
JH>  >     bar = foo;
JH>  >
JH>  > IMHO this is quite dangerous.
JH>  > Let's see:
JH>  >
JH>  >     thread1                 thread2
JH>  >     mtx_lock(&foo_mtx);
JH>  >     foo = data_from_user;
JH>  >                             bar = foo;
JH>  >     foo &= MASK;
JH>  >     mtx_unlock(&foo_mtx);
JH>  >
JH>  > In this case we have really dangerous race if data from user are
JH>  > safe only when we made 'and' operation on them.
JH>  > OR of course we can just store wrong value in 'bar' and this could
JH>  > be case of different problems.
JH>
JH>This case (along with some other cases where locks of atomic reads
JH>are required) is covered in the paper as
JH>
JH>  But, one case where locks would be required is if the field
JH>  temporarily holds a value that no one else is supposed to see and
JH>  the writer, operating with the lock held, will store a valid value
JH>  before releasing his lock. In this case, both the writer and
JH>  reader need to hold the lock before accessing this field.

Yes. When I read the C standard

	foo = data & mask;

wouldn't also help, because there is no sequence point in this statement
except at the ;. So the compiler is free to compile this as

	foo = data,
	foo &= mask;

unless foo is declared as volatile (in this case the write to foo is
supposed to have a side effect so that the compiler cannot write twice to
foo when only one write is specified). So I suppose that a lockless read
is (in MI code) only allowed if the object is 32-bit and is written too
with a simple assigment or is declared volatile.

harti
-- 
harti brandt,
http://www.fokus.fraunhofer.de/research/cc/cats/employees/hartmut.brandt/private
brandt at fokus.fraunhofer.de, harti at freebsd.org