Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 1/2] xen: fix race in xen_qlock_wait()
To:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        x86@kernel.org
Cc:     boris.ostrovsky@oracle.com, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, stable@vger.kernel.org,
        Waiman Long <longman@redhat.com>, peterz@infradead.org
References: <20181001071641.19282-1-jgross@suse.com>
 <20181001071641.19282-2-jgross@suse.com>
From:   Juergen Gross <jgross@suse.com>
Openpgp: preference=signencrypt
Autocrypt: addr=jgross@suse.com; prefer-encrypt=mutual; keydata=
 xsBNBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB
 ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve
 dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ
 NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx
 XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB
 AAHNHkp1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmRlPsLAeQQTAQIAIwUCU4xw6wIbAwcL
 CQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJELDendYovxMvi4UH/Ri+OXlObzqMANruTd4N
 zmVBAZgx1VW6jLc8JZjQuJPSsd/a+bNr3BZeLV6lu4Pf1Yl2Log129EX1KWYiFFvPbIiq5M5
 kOXTO8Eas4CaScCvAZ9jCMQCgK3pFqYgirwTgfwnPtxFxO/F3ZcS8jovza5khkSKL9JGq8Nk
 czDTruQ/oy0WUHdUr9uwEfiD9yPFOGqp4S6cISuzBMvaAiC5YGdUGXuPZKXLpnGSjkZswUzY
 d9BVSitRL5ldsQCg6GhDoEAeIhUC4SQnT9SOWkoDOSFRXZ+7+WIBGLiWMd+yKDdRG5RyP/8f
 3tgGiB6cyuYfPDRGsELGjUaTUq3H2xZgIPfOwE0EU4xwFgEIAMsx+gDjgzAY4H1hPVXgoLK8
 B93sTQFN9oC6tsb46VpxyLPfJ3T1A6Z6MVkLoCejKTJ3K9MUsBZhxIJ0hIyvzwI6aYJsnOew
 cCiCN7FeKJ/oA1RSUemPGUcIJwQuZlTOiY0OcQ5PFkV5YxMUX1F/aTYXROXgTmSaw0aC1Jpo
 w7Ss1mg4SIP/tR88/d1+HwkJDVW1RSxC1PWzGizwRv8eauImGdpNnseneO2BNWRXTJumAWDD
 pYxpGSsGHXuZXTPZqOOZpsHtInFyi5KRHSFyk2Xigzvh3b9WqhbgHHHE4PUVw0I5sIQt8hJq
 5nH5dPqz4ITtCL9zjiJsExHuHKN3NZsAEQEAAcLAXwQYAQIACQUCU4xwFgIbDAAKCRCw3p3W
 KL8TL0P4B/9YWver5uD/y/m0KScK2f3Z3mXJhME23vGBbMNlfwbr+meDMrJZ950CuWWnQ+d+
 Ahe0w1X7e3wuLVODzjcReQ/v7b4JD3wwHxe+88tgB9byc0NXzlPJWBaWV01yB2/uefVKryAf
 AHYEd0gCRhx7eESgNBe3+YqWAQawunMlycsqKa09dBDL1PFRosF708ic9346GLHRc6Vj5SRA
 UTHnQqLetIOXZm3a2eQ1gpQK9MmruO86Vo93p39bS1mqnLLspVrL4rhoyhsOyh0Hd28QCzpJ
 wKeHTd0MAWAirmewHXWPco8p1Wg+V+5xfZzuQY0f4tQxvOpXpt4gQ1817GQ5/Ed/wsDtBBgB
 CAAgFiEEhRJncuj2BJSl0Jf3sN6d1ii/Ey8FAlrd8NACGwIAgQkQsN6d1ii/Ey92IAQZFggA
 HRYhBFMtsHpB9jjzHji4HoBcYbtP2GO+BQJa3fDQAAoJEIBcYbtP2GO+TYsA/30H/0V6cr/W
 V+J/FCayg6uNtm3MJLo4rE+o4sdpjjsGAQCooqffpgA+luTT13YZNV62hAnCLKXH9n3+ZAgJ
 RtAyDWk1B/0SMDVs1wxufMkKC3Q/1D3BYIvBlrTVKdBYXPxngcRoqV2J77lscEvkLNUGsu/z
 W2pf7+P3mWWlrPMJdlbax00vevyBeqtqNKjHstHatgMZ2W0CFC4hJ3YEetuRBURYPiGzuJXU
 pAd7a7BdsqWC4o+GTm5tnGrCyD+4gfDSpkOT53S/GNO07YkPkm/8J4OBoFfgSaCnQ1izwgJQ
 jIpcG2fPCI2/hxf2oqXPYbKr1v4Z1wthmoyUgGN0LPTIm+B5vdY82wI5qe9uN6UOGyTH2B3p
 hRQUWqCwu2sqkI3LLbTdrnyDZaixT2T0f4tyF5Lfs+Ha8xVMhIyzNb1byDI5FKCb
Message-ID: <517372ba-4aad-d60c-6543-eb82942ebd20@suse.com>
Date:   Mon, 1 Oct 2018 09:37:37 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.0
MIME-Version: 1.0
In-Reply-To: <20181001071641.19282-2-jgross@suse.com>
Content-Type: text/plain; charset=utf-8
Content-Language: de-DE
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Correcting Waiman's mail address

On 01/10/2018 09:16, Juergen Gross wrote:
> In the following situation a vcpu waiting for a lock might not be
> woken up from xen_poll_irq():
> 
> CPU 1:                CPU 2:                      CPU 3:
> takes a spinlock
>                       tries to get lock
>                       -> xen_qlock_wait()
>                         -> xen_clear_irq_pending()
> frees the lock
> -> xen_qlock_kick(cpu2)
> 
> takes lock again
>                                                   tries to get lock
>                                                   -> *lock = _Q_SLOW_VAL
>                         -> *lock == _Q_SLOW_VAL ?
>                         -> xen_poll_irq()
> frees the lock
> -> xen_qlock_kick(cpu3)
> 
> And cpu 2 will sleep forever.
> 
> This can be avoided easily by modifying xen_qlock_wait() to call
> xen_poll_irq() only if the related irq was not pending and to call
> xen_clear_irq_pending() only if it was pending.
> 
> Cc: stable@vger.kernel.org
> Cc: longman@redhat.com
> Cc: peterz@infradead.org
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/xen/spinlock.c | 15 +++++----------
>  1 file changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 973f10e05211..cd210a4ba7b1 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8 val)
>  	if (irq == -1)
>  		return;
>  
> -	/* clear pending */
> -	xen_clear_irq_pending(irq);
> -	barrier();
> +	/* If irq pending already clear it and return. */
> +	if (xen_test_irq_pending(irq)) {
> +		xen_clear_irq_pending(irq);
> +		return;
> +	}
>  
> -	/*
> -	 * We check the byte value after clearing pending IRQ to make sure
> -	 * that we won't miss a wakeup event because of the clearing.
> -	 *
> -	 * The sync_clear_bit() call in xen_clear_irq_pending() is atomic.
> -	 * So it is effectively a memory barrier for x86.
> -	 */
>  	if (READ_ONCE(*byte) != val)
>  		return;
>  
>