Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

[Matthew Wilcox <willy@infradead.org>]

On Mon, Nov 11, 2024 at 12:55:05PM -0800, Suren Baghdasaryan wrote:
> When a reader takes read lock, it increments the atomic, unless the
> top two bits are set indicating a writer is present.
> When writer takes write lock, it sets VMA_LOCK_WR_LOCKED bit if there
> are no readers or VMA_LOCK_WR_WAIT bit if readers are holding the lock
> and puts itself onto newly introduced mm.vma_writer_wait. Since all
> writers take mmap_lock in write mode first, there can be only one writer
> at a time. The last reader to release the lock will signal the writer
> to wake up.

I don't think you need two bits.  You can do it this way:

0x8000'0000 - No readers, no writers
0x1-7fff'ffff - Some number of readers
0x0 - Writer held
0x8000'0001-0xffff'ffff - Reader held, writer waiting

A prospective writer subtracts 0x8000'0000.  If the result is 0, it got
the lock, otherwise it sleeps until it is 0.

A writer unlocks by adding 0x8000'0000 (not by setting the value to
0x8000'0000).

A reader unlocks by adding 1.  If the result is 0, it wakes the writer.

A prospective reader subtracts 1.  If the result is positive, it got the
lock, otherwise it does the unlock above (this might be the one which
wakes the writer).

And ... that's it.  See how we use the CPU arithmetic flags to tell us
everything we need to know without doing arithmetic separately?