linux-kernel - Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch

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Date:   Fri, 31 Aug 2018 00:42:46 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

On 08/30/2018 02:51 AM, Will Deacon wrote:
> Yeah, the bit_spin_lock()/__bit_spin_unlock() race described in f75d48644c56a
> boils down to concurrent atomic_long_set_release() vs
> atomic_long_fetch_or_acquire(), which really needs to work.

I don't see how: __clear_bit_unlock() reads @old, flips a bit and then calls
atomic_long_set_release() so the race is not just with set_release.

    static inline int test_and_set_bit_lock(unsigned int nr, volatile unsigned
long *p)
    {
      long old;
      unsigned long mask = (1UL << ((nr) % 32));

      p += ((nr) / 32);
      old = atomic_long_fetch_or_acquire(mask, (atomic_long_t *)p);
      return !!(old & mask);
    }

    static inline void __clear_bit_unlock(unsigned int nr, volatile unsigned long *p)
    {
       unsigned long old;

     p += ((nr) / 32);
    old = // soem typecheck magic on *p
    old &= ~(1UL << ((nr) % 32));
    atomic_long_set_release((atomic_long_t *)p, old);
  }