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Re: [Xen-devel] [PATCH 2/2] xen: make xen_qlock_wait() nestable


  • To: linux-kernel@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, x86@xxxxxxxxxx
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Mon, 1 Oct 2018 09:38:11 +0200
  • Autocrypt: addr=jgross@xxxxxxxx; 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
  • Cc: peterz@xxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx, mingo@xxxxxxxxxx, bp@xxxxxxxxx, hpa@xxxxxxxxx, Waiman Long <longman@xxxxxxxxxx>, boris.ostrovsky@xxxxxxxxxx, tglx@xxxxxxxxxxxxx
  • Delivery-date: Mon, 01 Oct 2018 07:38:14 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

Correcting Waiman's mail address

On 01/10/2018 09:16, Juergen Gross wrote:
> xen_qlock_wait() isn't safe for nested calls due to interrupts. A call
> of xen_qlock_kick() might be ignored in case a deeper nesting level
> was active right before the call of xen_poll_irq():
> 
> CPU 1:                                   CPU 2:
> spin_lock(lock1)
>                                          spin_lock(lock1)
>                                          -> xen_qlock_wait()
>                                             -> xen_clear_irq_pending()
>                                             Interrupt happens
> spin_unlock(lock1)
> -> xen_qlock_kick(CPU 2)
> spin_lock_irqsave(lock2)
>                                          spin_lock_irqsave(lock2)
>                                          -> xen_qlock_wait()
>                                             -> xen_clear_irq_pending()
>                                                clears kick for lock1
>                                             -> xen_poll_irq()
> spin_unlock_irq_restore(lock2)
> -> xen_qlock_kick(CPU 2)
>                                             wakes up
>                                          spin_unlock_irq_restore(lock2)
>                                          IRET
>                                            resumes in xen_qlock_wait()
>                                            -> xen_poll_irq()
>                                            never wakes up
> 
> The solution is to disable interrupts in xen_qlock_wait() and not to
> poll for the irq in case xen_qlock_wait() is called in nmi context.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: longman@xxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  arch/x86/xen/spinlock.c | 24 ++++++++++--------------
>  1 file changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index cd210a4ba7b1..e8d880e98057 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -39,29 +39,25 @@ static void xen_qlock_kick(int cpu)
>   */
>  static void xen_qlock_wait(u8 *byte, u8 val)
>  {
> +     unsigned long flags;
>       int irq = __this_cpu_read(lock_kicker_irq);
>  
>       /* If kicker interrupts not initialized yet, just spin */
> -     if (irq == -1)
> +     if (irq == -1 || in_nmi())
>               return;
>  
> -     /* If irq pending already clear it and return. */
> +     /* Guard against reentry. */
> +     local_irq_save(flags);
> +
> +     /* If irq pending already clear it. */
>       if (xen_test_irq_pending(irq)) {
>               xen_clear_irq_pending(irq);
> -             return;
> +     } else if (READ_ONCE(*byte) == val) {
> +             /* Block until irq becomes pending (or a spurious wakeup) */
> +             xen_poll_irq(irq);
>       }
>  
> -     if (READ_ONCE(*byte) != val)
> -             return;
> -
> -     /*
> -      * If an interrupt happens here, it will leave the wakeup irq
> -      * pending, which will cause xen_poll_irq() to return
> -      * immediately.
> -      */
> -
> -     /* Block until irq becomes pending (or perhaps a spurious wakeup) */
> -     xen_poll_irq(irq);
> +     local_irq_restore(flags);
>  }
>  
>  static irqreturn_t dummy_handler(int irq, void *dev_id)
> 


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