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Date: Wed, 15 Jul 2015 15:52:34 +0300
From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
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To: Eric Dumazet <eric.dumazet@gmail.com>
CC: Peter Zijlstra <peterz@infradead.org>, linux-arch@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org,
        Alexander Graf <agraf@suse.de>, Paul Mackerras <paulus@samba.org>,
        David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 3/3] sched/preempt: fix cond_resched_lock() and cond_resched_softirq()
References: <20150715095201.12246.49283.stgit@buzz>	 <20150715095204.12246.98268.stgit@buzz> <1436962596.1026.10.camel@edumazet-glaptop2.roam.corp.google.com>
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On 15.07.2015 15:16, Eric Dumazet wrote:
> On Wed, 2015-07-15 at 12:52 +0300, Konstantin Khlebnikov wrote:
>> These functions check should_resched() before unlocking spinlock/bh-enable:
>> preempt_count always non-zero => should_resched() always returns false.
>> cond_resched_lock() worked iff spin_needbreak is set.
>
> Interesting, this definitely used to work (linux-3.11)
>
> Any idea of which commit broke things ?
>

Searching... done

This one: bdb43806589096ac4272fe1307e789846ac08d7c in v3.13

before

-static inline int should_resched(void)
-{
-       return need_resched() && !(preempt_count() & PREEMPT_ACTIVE);
-}

after

+static __always_inline bool should_resched(void)
+{
+       return unlikely(!*preempt_count_ptr());
+}


So,

Fixes: bdb438065890 ("sched: Extract the basic add/sub preempt_count 
modifiers")

-- 
Konstantin