Re: [PATCH v4 4/6] hw_random: fix unregister race.
On Wed, Nov 12, 2014 at 02:33:00PM +1030, Rusty Russell wrote: > Amos Kong writes: > > From: Rusty Russell > > > > The previous patch added one potential problem: we can still be > > reading from a hwrng when it's unregistered. Add a wait for zero > > in the hwrng_unregister path. > > > > v4: add cleanup_done flag to insure that cleanup is done > > That's a bit weird. The usual pattern would be to hold a reference > until we're actually finished, but this reference is a bit weird. > > We hold the mutex across cleanup, so we could grab that but we have to > take care sleeping inside wait_event, otherwise Peter will have to fix > my code again :) > > AFAICT the wake_woken() stuff isn't merged yet, so your patch will > have to do for now. > > > @@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref) > > > > if (rng->cleanup) > > rng->cleanup(rng); > > + rng->cleanup_done = true; > > + wake_up_all(&rng_done); > > } > > > > static void set_current_rng(struct hwrng *rng) > > @@ -536,6 +539,11 @@ void hwrng_unregister(struct hwrng *rng) > > kthread_stop(hwrng_fill); > > } else > > mutex_unlock(&rng_mutex); > > + > > + /* Just in case rng is reading right now, wait. */ > > + wait_event(rng_done, rng->cleanup_done && > > + atomic_read(&rng->ref.refcount) == 0); > > + > > The atomic_read() isn't necessary here. At least, we need it to convert refcount from atomic_t to int. Otherwise, we will touch this error: error: invalid operands to binary == (have 'atomic_t' and 'int') > However, you should probably init cleanup_done in hwrng_register(). > (Probably noone does unregister then register, but let's be clear). > > Thanks, > Rusty. -- Amos. signature.asc Description: Digital signature ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [PATCH v4 4/6] hw_random: fix unregister race.
On Wed, Nov 12, 2014 at 02:33:00PM +1030, Rusty Russell wrote:
> Amos Kong writes:
> > From: Rusty Russell
> >
> > The previous patch added one potential problem: we can still be
> > reading from a hwrng when it's unregistered. Add a wait for zero
> > in the hwrng_unregister path.
> >
> > v4: add cleanup_done flag to insure that cleanup is done
>
> That's a bit weird. The usual pattern would be to hold a reference
> until we're actually finished, but this reference is a bit weird.
The cleanup function is a callback function of kref_put(), we can't
use the same reference count inside cleanup function.
> We hold the mutex across cleanup, so we could grab that but we have to
> take care sleeping inside wait_event, otherwise Peter will have to fix
> my code again :)
We didn't hold rng_mutex inside cleanup_rng(), am I missing something?
> AFAICT the wake_woken() stuff isn't merged yet, so your patch will
> have to do for now.
Can you provide some patches/mail link here? I searched nothing about
wake_woken.
> > @@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref)
> >
> > if (rng->cleanup)
> > rng->cleanup(rng);
> > + rng->cleanup_done = true;
> > + wake_up_all(&rng_done);
> > }
> >
> > static void set_current_rng(struct hwrng *rng)
> > @@ -536,6 +539,11 @@ void hwrng_unregister(struct hwrng *rng)
> > kthread_stop(hwrng_fill);
> > } else
> > mutex_unlock(&rng_mutex);
> > +
> > + /* Just in case rng is reading right now, wait. */
> > + wait_event(rng_done, rng->cleanup_done &&
> > + atomic_read(&rng->ref.refcount) == 0);
> > +
>
> The atomic_read() isn't necessary here.
>
> However, you should probably init cleanup_done in hwrng_register().
> (Probably noone does unregister then register, but let's be clear).
Got it.
> Thanks,
> Rusty.
>
> > }
> > EXPORT_SYMBOL_GPL(hwrng_unregister);
> >
> > diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
> > index c212e71..7832e50 100644
> > --- a/include/linux/hw_random.h
> > +++ b/include/linux/hw_random.h
> > @@ -46,6 +46,7 @@ struct hwrng {
> > /* internal. */
> > struct list_head list;
> > struct kref ref;
> > + bool cleanup_done;
> > };
> >
> > /** Register a new Hardware Random Number Generator driver. */
> > --
> > 1.9.3
--
Amos.
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Re: [PATCH v4 4/6] hw_random: fix unregister race.
On Mon, Nov 10, 2014 at 09:47:27PM +0800, Herbert Xu wrote: > On Mon, Nov 03, 2014 at 11:56:24PM +0800, Amos Kong wrote: > > > > @@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref) > > > > if (rng->cleanup) > > rng->cleanup(rng); > > You need a compiler barrier here to prevent reordering. Michael Büsch pointed out that we should actually have a memory barrier here. I thought we didn't need it because I was only worried about the code in my original complaint. However, expecting driver writers to use correct synchronisation primitives is surely asking too much. So let's add an smp_wmb() here to ensure all side-effects of cleanup is visible. Thanks, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [PATCH v4 4/6] hw_random: fix unregister race.
Amos Kong writes:
> From: Rusty Russell
>
> The previous patch added one potential problem: we can still be
> reading from a hwrng when it's unregistered. Add a wait for zero
> in the hwrng_unregister path.
>
> v4: add cleanup_done flag to insure that cleanup is done
That's a bit weird. The usual pattern would be to hold a reference
until we're actually finished, but this reference is a bit weird.
We hold the mutex across cleanup, so we could grab that but we have to
take care sleeping inside wait_event, otherwise Peter will have to fix
my code again :)
AFAICT the wake_woken() stuff isn't merged yet, so your patch will
have to do for now.
> @@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref)
>
> if (rng->cleanup)
> rng->cleanup(rng);
> + rng->cleanup_done = true;
> + wake_up_all(&rng_done);
> }
>
> static void set_current_rng(struct hwrng *rng)
> @@ -536,6 +539,11 @@ void hwrng_unregister(struct hwrng *rng)
> kthread_stop(hwrng_fill);
> } else
> mutex_unlock(&rng_mutex);
> +
> + /* Just in case rng is reading right now, wait. */
> + wait_event(rng_done, rng->cleanup_done &&
> +atomic_read(&rng->ref.refcount) == 0);
> +
The atomic_read() isn't necessary here.
However, you should probably init cleanup_done in hwrng_register().
(Probably noone does unregister then register, but let's be clear).
Thanks,
Rusty.
> }
> EXPORT_SYMBOL_GPL(hwrng_unregister);
>
> diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
> index c212e71..7832e50 100644
> --- a/include/linux/hw_random.h
> +++ b/include/linux/hw_random.h
> @@ -46,6 +46,7 @@ struct hwrng {
> /* internal. */
> struct list_head list;
> struct kref ref;
> + bool cleanup_done;
> };
>
> /** Register a new Hardware Random Number Generator driver. */
> --
> 1.9.3
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Re: [PATCH v4 4/6] hw_random: fix unregister race.
On Mon, Nov 03, 2014 at 11:56:24PM +0800, Amos Kong wrote: > > @@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref) > > if (rng->cleanup) > rng->cleanup(rng); You need a compiler barrier here to prevent reordering. > + rng->cleanup_done = true; Thanks, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
[PATCH v4 4/6] hw_random: fix unregister race.
From: Rusty Russell
The previous patch added one potential problem: we can still be
reading from a hwrng when it's unregistered. Add a wait for zero
in the hwrng_unregister path.
v4: add cleanup_done flag to insure that cleanup is done
Signed-off-by: Rusty Russell
Signed-off-by: Amos Kong
---
drivers/char/hw_random/core.c | 8
include/linux/hw_random.h | 1 +
2 files changed, 9 insertions(+)
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 27ad6b4..c31bf91 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -60,6 +60,7 @@ static DEFINE_MUTEX(rng_mutex);
static DEFINE_MUTEX(reading_mutex);
static int data_avail;
static u8 *rng_buffer, *rng_fillbuf;
+static DECLARE_WAIT_QUEUE_HEAD(rng_done);
static unsigned short current_quality;
static unsigned short default_quality; /* = 0; default to "off" */
@@ -98,6 +99,8 @@ static inline void cleanup_rng(struct kref *kref)
if (rng->cleanup)
rng->cleanup(rng);
+ rng->cleanup_done = true;
+ wake_up_all(&rng_done);
}
static void set_current_rng(struct hwrng *rng)
@@ -536,6 +539,11 @@ void hwrng_unregister(struct hwrng *rng)
kthread_stop(hwrng_fill);
} else
mutex_unlock(&rng_mutex);
+
+ /* Just in case rng is reading right now, wait. */
+ wait_event(rng_done, rng->cleanup_done &&
+ atomic_read(&rng->ref.refcount) == 0);
+
}
EXPORT_SYMBOL_GPL(hwrng_unregister);
diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index c212e71..7832e50 100644
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -46,6 +46,7 @@ struct hwrng {
/* internal. */
struct list_head list;
struct kref ref;
+ bool cleanup_done;
};
/** Register a new Hardware Random Number Generator driver. */
--
1.9.3
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