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From: Julien Grall <julien@xen.org>
To: Roger Pau Monne <roger.pau@citrix.com>, xen-devel@lists.xenproject.org
Cc: Igor Druzhinin <igor.druzhinin@citrix.com>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH] xen/spinlock: move debug helpers inside the locked regions
Date: Wed, 29 Jul 2020 14:37:44 +0100	[thread overview]
Message-ID: <16dd0f04-598b-8b84-8a25-6b89af9214d7@xen.org> (raw)
In-Reply-To: <20200729111330.64549-1-roger.pau@citrix.com>

Hi Roger,

On 29/07/2020 12:13, Roger Pau Monne wrote:
> Debug helpers such as lock profiling or the invariant pCPU assertions
> must strictly be performed inside the exclusive locked region, or else
> races might happen.
> 
> Note the issue was not strictly introduced by the pointed commit in
> the Fixes tag, since lock stats where already incremented before the
> barrier, but that commit made it more apparent as manipulating the cpu
> field could happen outside of the locked regions and thus trigger the
> BUG_ON.

 From the wording, it is not entirely clear which BUG_ON() you are 
referring to. I am guessing, it is the one in rel_lock(). Am I correct?

Otherwise, the change looks good to me.

Cheers,

> This is only enabled on debug builds, and thus releases are
> not affected.
> 
> Fixes: 80cba391a35 ('spinlocks: in debug builds store cpu holding the lock')
> Reported-by: Igor Druzhinin <igor.druzhinin@citrix.com>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
>   xen/common/spinlock.c | 12 ++++++------
>   1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
> index 17f4519fc7..ce3106e2d3 100644
> --- a/xen/common/spinlock.c
> +++ b/xen/common/spinlock.c
> @@ -170,9 +170,9 @@ void inline _spin_lock_cb(spinlock_t *lock, void (*cb)(void *), void *data)
>               cb(data);
>           arch_lock_relax();
>       }
> +    arch_lock_acquire_barrier();
>       got_lock(&lock->debug);
>       LOCK_PROFILE_GOT;
> -    arch_lock_acquire_barrier();
>   }
>   
>   void _spin_lock(spinlock_t *lock)
> @@ -198,9 +198,9 @@ unsigned long _spin_lock_irqsave(spinlock_t *lock)
>   
>   void _spin_unlock(spinlock_t *lock)
>   {
> -    arch_lock_release_barrier();
>       LOCK_PROFILE_REL;
>       rel_lock(&lock->debug);
> +    arch_lock_release_barrier();
>       add_sized(&lock->tickets.head, 1);
>       arch_lock_signal();
>       preempt_enable();
> @@ -249,15 +249,15 @@ int _spin_trylock(spinlock_t *lock)
>           preempt_enable();
>           return 0;
>       }
> +    /*
> +     * cmpxchg() is a full barrier so no need for an
> +     * arch_lock_acquire_barrier().
> +     */
>       got_lock(&lock->debug);
>   #ifdef CONFIG_DEBUG_LOCK_PROFILE
>       if (lock->profile)
>           lock->profile->time_locked = NOW();
>   #endif
> -    /*
> -     * cmpxchg() is a full barrier so no need for an
> -     * arch_lock_acquire_barrier().
> -     */
>       return 1;
>   }
>   
> 

-- 
Julien Grall


  reply	other threads:[~2020-07-29 13:38 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-29 11:13 [PATCH] xen/spinlock: move debug helpers inside the locked regions Roger Pau Monne
2020-07-29 13:37 ` Julien Grall [this message]
2020-07-29 13:50   ` Roger Pau Monné
2020-07-29 14:57     ` Julien Grall
2020-07-30 18:29       ` Julien Grall

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