msdosfs not MPSAFE

[Bruce Evans]

On Fri, 10 Aug 2007, Kostik Belousov wrote:

> On Fri, Aug 10, 2007 at 01:54:48PM +1000, Bruce Evans wrote:
>> I wrote yet another patch, with allocation on the stack so that no locking
>> is required.  This is simpler and doesn't require any new functions.
>> Unfortunately, it is larger because it changes the interfaces for most
>> functions.  The interface changes are routine, so this is probably better.
>> Note that 'struct dirent's are already allocated on the stack.  This
>> patch adds allocation of 'struct mbnambuf's which are slightly smaller
>> (~256 bytes).  I think this is just small enough for stack allocation.
>
> I agree that this is the best approach. The size of the on-stack
> structure still make me worry, although ~270 bytes seems to be not too
> large for 3-pages stack.

Stack growth seems to be nowhere near a problem.  With the extra ~270
bytes.  ls -lR on i386 uses less than a 1-page stack (about 3.5K,
including 0x270 bytes for the pcb).  I think the maximum stack depth
is attained when a debugger trap traps a fast interrupt interrupting
a page fault in an i/o routine called from msdosfs_readdir().  Unlike
in RELENG_4, nested interrupts can't occur, so a 1-page stack should
be enough for everything on i386 (but of course isn't quite enough).
I didn't try hard to attain the maximum depth, and just looked at where
the stack got to after running a large ls -lR for a while.  msdosfs_readdir()
now allocates 0x2b0 bytes on the stack using "subl $0x2b0,%esp" and
that is now the largest single allocation.  This is without INVARIANTS
etc.

Bruce