locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type

Partially revert commit 0aaddfb06882 ("locking/local_lock: Introduce
localtry_lock_t").  Remove localtry_*() helpers, since localtry_lock()
name might be misinterpreted as "try lock".

Introduce local_trylock[_irqsave]() helpers that only work with newly
introduced local_trylock_t type.  Note that attempt to use
local_trylock[_irqsave]() with local_lock_t will cause compilation
failure.

Usage and behavior in !PREEMPT_RT:

local_lock_t lock;                     // sizeof(lock) == 0
local_lock(&lock);                     // preempt disable
local_lock_irqsave(&lock, ...);        // irq save
if (local_trylock_irqsave(&lock, ...)) // compilation error

local_trylock_t lock;                  // sizeof(lock) == 4
local_lock(&lock);                     // preempt disable, acquired = 1
local_lock_irqsave(&lock, ...);        // irq save, acquired = 1
if (local_trylock(&lock))              // if (!acquired) preempt disable, acquired = 1
if (local_trylock_irqsave(&lock, ...)) // if (!acquired) irq save, acquired = 1

The existing local_lock_*() macros can be used either with local_lock_t or
local_trylock_t.  With local_trylock_t they set acquired = 1 while
local_unlock_*() clears it.

In !PREEMPT_RT local_lock_irqsave(local_lock_t *) disables interrupts to
protect critical section, but it doesn't prevent NMI, so the fully
reentrant code cannot use local_lock_irqsave(local_lock_t *) for exclusive
access.

The local_lock_irqsave(local_trylock_t *) helper disables interrupts and
sets acquired=1, so local_trylock_irqsave(local_trylock_t *) from NMI
attempting to acquire the same lock will return false.

In PREEMPT_RT local_lock_irqsave() maps to preemptible spin_lock().  Map
local_trylock_irqsave() to preemptible spin_trylock().  When in hard IRQ
or NMI return false right away, since spin_trylock() is not safe due to
explicit locking in the underneath rt_spin_trylock() implementation. 
Removing this explicit locking and attempting only "trylock" is undesired
due to PI implications.

The local_trylock() without _irqsave can be used to avoid the cost of
disabling/enabling interrupts by only disabling preemption, so
local_trylock() in an interrupt attempting to acquire the same lock will
return false.

Note there is no need to use local_inc for acquired variable, since it's a
percpu variable with strict nesting scopes.

Note that guard(local_lock)(&lock) works only for "local_lock_t lock".

The patch also makes sure that local_lock_release(l) is called before
WRITE_ONCE(l->acquired, 0).  Though IRQs are disabled at this point the
local_trylock() from NMI will succeed and local_lock_acquire(l) will warn.

Link: https://lkml.kernel.org/r/20250403025514.41186-1-alexei.starovoitov@gmail.com
Fixes: 0aaddfb06882 ("locking/local_lock: Introduce localtry_lock_t")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Daniel Borkman <daniel@iogearbox.net>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions