Re: [PATCH] fat: Prevent the race of read/write the FAT16 and FAT32 entry

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Jul 29, 2025 at 05:53:23AM +0100, Al Viro wrote:

> FAT12 problem is that FAT entries being accessed there are 12-bit, packed in
> pairs into an array of 3-byte values.

PS: they most definitely can cross the cacheline boundaries - cacheline size
is not going to be a multiple of 3 on anything realistic.  Hell, they can
cross *block* boundaries (which is why for FAT12 that code is using two
separate pointers - most of the time they point to adjacent bytes, but
if one byte is in one block and the next one is in another...; that's
what that if in fat12_ent_set_ptr() is doing)...

We could map the entire array contiguously (and it might simplify some of
the logics there), but it's not going to avoid the problem with a single
entry occupying a byte in one cacheline and half of a byte in another.

So nothing like cmpxchg would suffice - we need a spinlock for FAT12 case.