mmap_lock nests under uring_lock out of necessity, as we may be doing
user copies with uring_lock held. However, for mmap of provided buffer
rings, we attempt to grab uring_lock with mmap_lock already held from
do_mmap(). This makes lockdep, rightfully, complain:

WARNING: possible circular locking dependency detected
6.7.0-rc1-00009-gff3337ebaf94-dirty #4438 Not tainted
------------------------------------------------------
buf-ring.t/442 is trying to acquire lock:
ffff00020e1480a8 (&ctx->uring_lock){+.+.}-{3:3}, at: io_uring_validate_mmap_request.isra.0+0x4c/0x140

but task is already holding lock:
ffff0000dc226190 (&mm->mmap_lock){++++}-{3:3}, at: vm_mmap_pgoff+0x124/0x264

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&mm->mmap_lock){++++}-{3:3}:
       __might_fault+0x90/0xbc
       io_register_pbuf_ring+0x94/0x488
       __arm64_sys_io_uring_register+0x8dc/0x1318
       invoke_syscall+0x5c/0x17c
       el0_svc_common.constprop.0+0x108/0x130
       do_el0_svc+0x2c/0x38
       el0_svc+0x4c/0x94
       el0t_64_sync_handler+0x118/0x124
       el0t_64_sync+0x168/0x16c

-> #0 (&ctx->uring_lock){+.+.}-{3:3}:
       __lock_acquire+0x19a0/0x2d14
       lock_acquire+0x2e0/0x44c
       __mutex_lock+0x118/0x564
       mutex_lock_nested+0x20/0x28
       io_uring_validate_mmap_request.isra.0+0x4c/0x140
       io_uring_mmu_get_unmapped_area+0x3c/0x98
       get_unmapped_area+0xa4/0x158
       do_mmap+0xec/0x5b4
       vm_mmap_pgoff+0x158/0x264
       ksys_mmap_pgoff+0x1d4/0x254
       __arm64_sys_mmap+0x80/0x9c
       invoke_syscall+0x5c/0x17c
       el0_svc_common.constprop.0+0x108/0x130
       do_el0_svc+0x2c/0x38
       el0_svc+0x4c/0x94
       el0t_64_sync_handler+0x118/0x124
       el0t_64_sync+0x168/0x16c

From that mmap(2) path, we really just need to ensure that the buffer
list doesn't go away from underneath us. For the lower indexed entries,
they never go away until the ring is freed and we can always sanely
reference those as long as the caller has a file reference. For the
higher indexed ones in our xarray, we just need to ensure that the
buffer list remains valid while we return the address of it.

Free the higher indexed io_buffer_list entries via RCU. With that we can
avoid needing ->uring_lock inside mmap(2), and simply hold the RCU read
lock around the buffer list lookup and address check.

To ensure that the arrayed lookup either returns a valid fully formulated
entry via RCU lookup, add an 'is_ready' flag that we access with store
and release memory ordering. This isn't needed for the xarray lookups,
but doesn't hurt either. Since this isn't a fast path, retain it across
both types. Similarly, for the allocated array inside the ctx, ensure
we use the proper load/acquire as setup could in theory be running in
parallel with mmap.

While in there, add a few lockdep checks for documentation purposes.

Cc: stable@vger.kernel.org
Fixes: c56e022c ("io_uring: add support for user mapped provided buffer ring")
Signed-off-by: Jens Axboe <axboe@kernel.dk>