linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Message-ID: <20200813143221.GI17456@casper.infradead.org>
Date:   Thu, 13 Aug 2020 15:32:21 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Thu, Aug 13, 2020 at 03:27:15PM +0200, Thomas Gleixner wrote:
> And guarding it with RT is not working either because then you are back
> to square one with the problem which triggered the discussion in the
> first place:
> 
> raw_spin_lock()
>   alloc()
>     if (RT && !preemptible())  <- False because RT == false
>     	goto bail;
> 
>     spin_lock(&zone->lock)  --> LOCKDEP complains
> 
> So either you convince Paul not to do that or you need to do something
> like I suggested in my other reply.

I'd like to throw in the possibility that we do something like:

  raw_spin_lock()
    alloc()
      if (!spin_trylock(&zone->lock))
        if (RT && !preemptible())
          goto bail;
        spin_lock(&zone->lock);

would that make us feel more comfortable about converting zone->lock to
a raw spinlock?