linux-kernel - Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <Y8bodcnhyMox+QjG@casper.infradead.org>
Date:   Tue, 17 Jan 2023 18:27:01 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Hillf Danton <hdanton@...a.com>, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net,
        peterz@...radead.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock

On Tue, Jan 17, 2023 at 10:21:28AM -0800, Suren Baghdasaryan wrote:
> static inline bool vma_read_trylock(struct vm_area_struct *vma)
> {
>        int count, new;
> 
>         /* Check before locking. A race might cause false locked result. */
>        if (READ_ONCE(vma->vm_lock->lock_seq) ==
>            READ_ONCE(vma->vm_mm->mm_lock_seq))
>                 return false;
> 
>         count = atomic_read(&vma->vm_lock->count);
>         for (;;) {
>               /*
>                * Is VMA is write-locked? Overflow might produce false
> locked result.
>                * False unlocked result is impossible because we modify and check
>                * vma->vm_lock_seq under vma->vm_lock protection and
> mm->mm_lock_seq
>                * modification invalidates all existing locks.
>                */
>               if (count < 0)
>                         return false;
> 
>              new = count + 1;
>              /* If atomic_t overflows, fail to lock. */
>              if (new < 0)
>                         return false;
> 
>              /*
>               * Atomic RMW will provide implicit mb on success to pair
> with smp_wmb in
>               * vma_write_lock, on failure we retry.
>               */
>               new = atomic_cmpxchg(&vma->vm_lock->count, count, new);
>                 if (new == count)
>                         break;
>                 count = new;
>                 cpu_relax();

The cpu_relax() is exactly the wrong thing to do here.  See this thread:
https://lore.kernel.org/linux-fsdevel/20230113184447.1707316-1-mjguzik@gmail.com/