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Date:   Fri, 23 Nov 2018 12:02:55 +0100
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     zhe.he@windriver.com, catalin.marinas@arm.com, tglx@linutronix.de,
        rostedt@goodmis.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        boqun.feng@gmail.com
Subject: Re: [PATCH v2] kmemleak: Turn kmemleak_lock to raw spinlock on RT
Message-ID: <20181123110226.GA5125@andrea>
References: <1542877459-144382-1-git-send-email-zhe.he@windriver.com>
 <20181123095314.hervxkxtqoixovro@linutronix.de>
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> is this an RT-only problem? Because mainline should not allow read->read
> locking or read->write locking for reader-writer locks. If this only
> happens on v4.18 and not on v4.19 then something must have fixed it.

Probably misunderstanding, but I'd say that read->read locking is "the
norm"...?

If you don't use qrwlock, readers are also "recursive", in part.,

  P0			P1
  read_lock(l)
			write_lock(l)
  read_lock(l)

won't block P0 on the second read_lock().  (qrwlock somehow complicate
the analysis; IIUC, they are recursive if and only if in_interrupt().).

  Andrea


>  
> 
> Sebastian