* mutex
@ 2014-09-09 15:18 Amos Kong
2014-09-09 15:23 ` RFC virtio-rng: fail to read sysfs of a busy device Amos Kong
0 siblings, 1 reply; 5+ messages in thread
From: Amos Kong @ 2014-09-09 15:18 UTC (permalink / raw)
To: virtualization; +Cc: amit.shah, herbert, kvm
Hi Amit, Rusty
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1127062
steps:
- Read random data by 'dd if=/dev/hwrng of=/dev/null' in guest
- check sysfs files in the same time, 'cat /sys/class/misc/hw_random/rng_*'
Result: cat process will get stuck, it will return if we kill dd process.
We have some static variables (eg, current_rng, data_avail, etc) in hw_random/core.c,
they are protected by rng_mutex. I try to workaround this issue by undelay(100)
after mutex_unlock() in rng_dev_read(). This gives chance for hwrng_attr_*_show()
to get mutex.
This patch also contains some cleanup, moving some code out of mutex
protection.
Do you have some suggestion? Thanks.
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index aa30a25..fa69020 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -194,6 +194,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
}
mutex_unlock(&rng_mutex);
+ udelay(100);
if (need_resched())
schedule_timeout_interruptible(1);
@@ -233,10 +234,10 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
int err;
struct hwrng *rng;
+ err = -ENODEV;
err = mutex_lock_interruptible(&rng_mutex);
if (err)
return -ERESTARTSYS;
- err = -ENODEV;
list_for_each_entry(rng, &rng_list, list) {
if (strcmp(rng->name, buf) == 0) {
if (rng == current_rng) {
@@ -270,8 +271,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
return -ERESTARTSYS;
if (current_rng)
name = current_rng->name;
- ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
mutex_unlock(&rng_mutex);
+ ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
return ret;
}
@@ -284,19 +285,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
ssize_t ret = 0;
struct hwrng *rng;
+ buf[0] = '\0';
err = mutex_lock_interruptible(&rng_mutex);
if (err)
return -ERESTARTSYS;
- buf[0] = '\0';
list_for_each_entry(rng, &rng_list, list) {
strncat(buf, rng->name, PAGE_SIZE - ret - 1);
ret += strlen(rng->name);
strncat(buf, " ", PAGE_SIZE - ret - 1);
ret++;
}
+ mutex_unlock(&rng_mutex);
strncat(buf, "\n", PAGE_SIZE - ret - 1);
ret++;
- mutex_unlock(&rng_mutex);
return ret;
}
^ permalink raw reply related [flat|nested] 5+ messages in thread
* RFC virtio-rng: fail to read sysfs of a busy device
2014-09-09 15:18 mutex Amos Kong
@ 2014-09-09 15:23 ` Amos Kong
2014-09-10 5:52 ` Amit Shah
0 siblings, 1 reply; 5+ messages in thread
From: Amos Kong @ 2014-09-09 15:23 UTC (permalink / raw)
To: virtualization; +Cc: amit.shah, herbert, kvm
(Resend to fix the subject)
Hi Amit, Rusty
RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1127062
steps:
- Read random data by 'dd if=/dev/hwrng of=/dev/null' in guest
- check sysfs files in the same time, 'cat /sys/class/misc/hw_random/rng_*'
Result: cat process will get stuck, it will return if we kill dd process.
We have some static variables (eg, current_rng, data_avail, etc) in hw_random/core.c,
they are protected by rng_mutex. I try to workaround this issue by undelay(100)
after mutex_unlock() in rng_dev_read(). This gives chance for hwrng_attr_*_show()
to get mutex.
This patch also contains some cleanup, moving some code out of mutex
protection.
Do you have some suggestion? Thanks.
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index aa30a25..fa69020 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -194,6 +194,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
}
mutex_unlock(&rng_mutex);
+ udelay(100);
if (need_resched())
schedule_timeout_interruptible(1);
@@ -233,10 +234,10 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
int err;
struct hwrng *rng;
+ err = -ENODEV;
err = mutex_lock_interruptible(&rng_mutex);
if (err)
return -ERESTARTSYS;
- err = -ENODEV;
list_for_each_entry(rng, &rng_list, list) {
if (strcmp(rng->name, buf) == 0) {
if (rng == current_rng) {
@@ -270,8 +271,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
return -ERESTARTSYS;
if (current_rng)
name = current_rng->name;
- ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
mutex_unlock(&rng_mutex);
+ ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
return ret;
}
@@ -284,19 +285,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
ssize_t ret = 0;
struct hwrng *rng;
+ buf[0] = '\0';
err = mutex_lock_interruptible(&rng_mutex);
if (err)
return -ERESTARTSYS;
- buf[0] = '\0';
list_for_each_entry(rng, &rng_list, list) {
strncat(buf, rng->name, PAGE_SIZE - ret - 1);
ret += strlen(rng->name);
strncat(buf, " ", PAGE_SIZE - ret - 1);
ret++;
}
+ mutex_unlock(&rng_mutex);
strncat(buf, "\n", PAGE_SIZE - ret - 1);
ret++;
- mutex_unlock(&rng_mutex);
return ret;
}
--
Amos.
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: RFC virtio-rng: fail to read sysfs of a busy device
2014-09-09 15:23 ` RFC virtio-rng: fail to read sysfs of a busy device Amos Kong
@ 2014-09-10 5:52 ` Amit Shah
2014-09-10 6:49 ` Amos Kong
0 siblings, 1 reply; 5+ messages in thread
From: Amit Shah @ 2014-09-10 5:52 UTC (permalink / raw)
To: Amos Kong; +Cc: herbert, kvm, virtualization
On (Tue) 09 Sep 2014 [23:23:07], Amos Kong wrote:
> (Resend to fix the subject)
>
> Hi Amit, Rusty
>
> RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1127062
> steps:
> - Read random data by 'dd if=/dev/hwrng of=/dev/null' in guest
> - check sysfs files in the same time, 'cat /sys/class/misc/hw_random/rng_*'
>
> Result: cat process will get stuck, it will return if we kill dd process.
How common is it going to be to have a long-running 'dd' process on
/dev/hwrng?
Also, with the new khwrng thread, reading from /dev/hwrng isn't
required -- just use /dev/random?
(This doesn't mean we shouldn't fix the issue here...)
> We have some static variables (eg, current_rng, data_avail, etc) in hw_random/core.c,
> they are protected by rng_mutex. I try to workaround this issue by undelay(100)
> after mutex_unlock() in rng_dev_read(). This gives chance for hwrng_attr_*_show()
> to get mutex.
>
> This patch also contains some cleanup, moving some code out of mutex
> protection.
>
> Do you have some suggestion? Thanks.
>
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index aa30a25..fa69020 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -194,6 +194,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
> }
>
> mutex_unlock(&rng_mutex);
> + udelay(100);
We have a need_resched() right below. Why doesn't that work?
> if (need_resched())
> schedule_timeout_interruptible(1);
> @@ -233,10 +234,10 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
> int err;
> struct hwrng *rng;
The following hunk doesn't work:
> + err = -ENODEV;
> err = mutex_lock_interruptible(&rng_mutex);
err is being set to another value in the next line!
> if (err)
> return -ERESTARTSYS;
> - err = -ENODEV;
And all usage of err below now won't have -ENODEV but some other value.
> list_for_each_entry(rng, &rng_list, list) {
> if (strcmp(rng->name, buf) == 0) {
> if (rng == current_rng) {
> @@ -270,8 +271,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
> return -ERESTARTSYS;
> if (current_rng)
> name = current_rng->name;
> - ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> mutex_unlock(&rng_mutex);
> + ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
This looks OK...
>
> return ret;
> }
> @@ -284,19 +285,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> ssize_t ret = 0;
> struct hwrng *rng;
>
> + buf[0] = '\0';
> err = mutex_lock_interruptible(&rng_mutex);
> if (err)
> return -ERESTARTSYS;
>
> - buf[0] = '\0';
> list_for_each_entry(rng, &rng_list, list) {
> strncat(buf, rng->name, PAGE_SIZE - ret - 1);
> ret += strlen(rng->name);
> strncat(buf, " ", PAGE_SIZE - ret - 1);
> ret++;
> }
> + mutex_unlock(&rng_mutex);
> strncat(buf, "\n", PAGE_SIZE - ret - 1);
> ret++;
> - mutex_unlock(&rng_mutex);
But this isn't resulting in savings; the majority of the time is being
spent in the for loop, and that writes to the buffer.
BTW I don't expect strcat'ing to the buf in each of these scenarios is
a long operation, so this reworking doesn't strike to me as something
we should pursue.
Amit
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RFC virtio-rng: fail to read sysfs of a busy device
2014-09-10 5:52 ` Amit Shah
@ 2014-09-10 6:49 ` Amos Kong
2014-09-10 7:32 ` Amos Kong
0 siblings, 1 reply; 5+ messages in thread
From: Amos Kong @ 2014-09-10 6:49 UTC (permalink / raw)
To: Amit Shah; +Cc: herbert, kvm, virtualization
On Wed, Sep 10, 2014 at 11:22:12AM +0530, Amit Shah wrote:
> On (Tue) 09 Sep 2014 [23:23:07], Amos Kong wrote:
> > (Resend to fix the subject)
> >
> > Hi Amit, Rusty
> >
> > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1127062
> > steps:
> > - Read random data by 'dd if=/dev/hwrng of=/dev/null' in guest
> > - check sysfs files in the same time, 'cat /sys/class/misc/hw_random/rng_*'
> >
> > Result: cat process will get stuck, it will return if we kill dd process.
>
> How common is it going to be to have a long-running 'dd' process on
> /dev/hwrng?
Not a common usage, but we have this strict testing.
> Also, with the new khwrng thread, reading from /dev/hwrng isn't
> required -- just use /dev/random?
Yes.
> (This doesn't mean we shouldn't fix the issue here...)
Completely agree :-)
> > We have some static variables (eg, current_rng, data_avail, etc) in hw_random/core.c,
> > they are protected by rng_mutex. I try to workaround this issue by undelay(100)
> > after mutex_unlock() in rng_dev_read(). This gives chance for hwrng_attr_*_show()
> > to get mutex.
> >
> > This patch also contains some cleanup, moving some code out of mutex
> > protection.
> >
> > Do you have some suggestion? Thanks.
> >
> >
> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > index aa30a25..fa69020 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -194,6 +194,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
> > }
> >
> > mutex_unlock(&rng_mutex);
> > + udelay(100);
>
> We have a need_resched() right below. Why doesn't that work?
need_resched() is giving chance for userspace to
> > if (need_resched())
It never success in my debugging.
If we remove this check and always call schedule_timeout_interruptible(1),
problem also disappears.
diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index aa30a25..263a370 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -195,8 +195,7 @@ static ssize_t rng_dev_read(struct file *filp,
char __user *buf,
mutex_unlock(&rng_mutex);
- if (need_resched())
- schedule_timeout_interruptible(1);
+ schedule_timeout_interruptible(1);
if (signal_pending(current)) {
err = -ERESTARTSYS;
> > schedule_timeout_interruptible(1);
> > @@ -233,10 +234,10 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
> > int err;
> > struct hwrng *rng;
> The following hunk doesn't work:
>
> > + err = -ENODEV;
> > err = mutex_lock_interruptible(&rng_mutex);
>
> err is being set to another value in the next line!
>
> > if (err)
> > return -ERESTARTSYS;
> > - err = -ENODEV;
>
> And all usage of err below now won't have -ENODEV but some other value.
Oops!
> > list_for_each_entry(rng, &rng_list, list) {
> > if (strcmp(rng->name, buf) == 0) {
> > if (rng == current_rng) {
> > @@ -270,8 +271,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
> > return -ERESTARTSYS;
> > if (current_rng)
> > name = current_rng->name;
> > - ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> > mutex_unlock(&rng_mutex);
> > + ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
>
> This looks OK...
>
> >
> > return ret;
> > }
> > @@ -284,19 +285,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> > ssize_t ret = 0;
> > struct hwrng *rng;
> >
> > + buf[0] = '\0';
> > err = mutex_lock_interruptible(&rng_mutex);
> > if (err)
> > return -ERESTARTSYS;
> >
> > - buf[0] = '\0';
> > list_for_each_entry(rng, &rng_list, list) {
> > strncat(buf, rng->name, PAGE_SIZE - ret - 1);
> > ret += strlen(rng->name);
> > strncat(buf, " ", PAGE_SIZE - ret - 1);
> > ret++;
> > }
> > + mutex_unlock(&rng_mutex);
> > strncat(buf, "\n", PAGE_SIZE - ret - 1);
> > ret++;
> > - mutex_unlock(&rng_mutex);
>
> But this isn't resulting in savings; the majority of the time is being
> spent in the for loop, and that writes to the buffer.
Right
> BTW I don't expect strcat'ing to the buf in each of these scenarios is
> a long operation, so this reworking doesn't strike to me as something
> we should pursue.
>
> Amit
--
Amos.
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: RFC virtio-rng: fail to read sysfs of a busy device
2014-09-10 6:49 ` Amos Kong
@ 2014-09-10 7:32 ` Amos Kong
0 siblings, 0 replies; 5+ messages in thread
From: Amos Kong @ 2014-09-10 7:32 UTC (permalink / raw)
To: Amit Shah; +Cc: herbert, kvm, virtualization
On Wed, Sep 10, 2014 at 02:49:38PM +0800, Amos Kong wrote:
> On Wed, Sep 10, 2014 at 11:22:12AM +0530, Amit Shah wrote:
> > On (Tue) 09 Sep 2014 [23:23:07], Amos Kong wrote:
> > > (Resend to fix the subject)
> > >
> > > Hi Amit, Rusty
> > >
> > > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1127062
> > > steps:
> > > - Read random data by 'dd if=/dev/hwrng of=/dev/null' in guest
> > > - check sysfs files in the same time, 'cat /sys/class/misc/hw_random/rng_*'
> > >
> > > Result: cat process will get stuck, it will return if we kill dd process.
> >
> > How common is it going to be to have a long-running 'dd' process on
> > /dev/hwrng?
>
> Not a common usage, but we have this strict testing.
For -smp 1:
It's easy to reproduce with slow backend (/dev/random). cat can return
most of time with some delay if we use quick backend (/dev/urandom).
But for -smp 2:
I didn't touch this problem even with slow backend.
> > Also, with the new khwrng thread, reading from /dev/hwrng isn't
> > required -- just use /dev/random?
>
> Yes.
>
> > (This doesn't mean we shouldn't fix the issue here...)
>
> Completely agree :-)
>
> > > We have some static variables (eg, current_rng, data_avail, etc) in hw_random/core.c,
> > > they are protected by rng_mutex. I try to workaround this issue by undelay(100)
> > > after mutex_unlock() in rng_dev_read(). This gives chance for hwrng_attr_*_show()
> > > to get mutex.
> > >
> > > This patch also contains some cleanup, moving some code out of mutex
> > > protection.
> > >
> > > Do you have some suggestion? Thanks.
> > >
> > >
> > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > > index aa30a25..fa69020 100644
> > > --- a/drivers/char/hw_random/core.c
> > > +++ b/drivers/char/hw_random/core.c
> > > @@ -194,6 +194,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
> > > }
> > >
> > > mutex_unlock(&rng_mutex);
> > > + udelay(100);
> >
> > We have a need_resched() right below. Why doesn't that work?
[smp 1]
Why need_resched() always return zero? what's the original purpose of it ?
>
> > > if (need_resched())
>
> It never success in my debugging.
>
> If we remove this check and always call schedule_timeout_interruptible(1),
> problem also disappears.
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index aa30a25..263a370 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -195,8 +195,7 @@ static ssize_t rng_dev_read(struct file *filp,
> char __user *buf,
>
> mutex_unlock(&rng_mutex);
>
> - if (need_resched())
> - schedule_timeout_interruptible(1);
> + schedule_timeout_interruptible(1);
>
> if (signal_pending(current)) {
> err = -ERESTARTSYS;
>
> > > schedule_timeout_interruptible(1);
> > > @@ -233,10 +234,10 @@ static ssize_t hwrng_attr_current_store(struct device *dev,
> > > int err;
> > > struct hwrng *rng;
>
> > The following hunk doesn't work:
> >
> > > + err = -ENODEV;
> > > err = mutex_lock_interruptible(&rng_mutex);
> >
> > err is being set to another value in the next line!
> >
> > > if (err)
> > > return -ERESTARTSYS;
> > > - err = -ENODEV;
> >
> > And all usage of err below now won't have -ENODEV but some other value.
>
> Oops!
>
> > > list_for_each_entry(rng, &rng_list, list) {
> > > if (strcmp(rng->name, buf) == 0) {
> > > if (rng == current_rng) {
> > > @@ -270,8 +271,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
> > > return -ERESTARTSYS;
> > > if (current_rng)
> > > name = current_rng->name;
> > > - ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> > > mutex_unlock(&rng_mutex);
> > > + ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> >
> > This looks OK...
> >
> > >
> > > return ret;
> > > }
> > > @@ -284,19 +285,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> > > ssize_t ret = 0;
> > > struct hwrng *rng;
> > >
> > > + buf[0] = '\0';
> > > err = mutex_lock_interruptible(&rng_mutex);
> > > if (err)
> > > return -ERESTARTSYS;
> > >
> > > - buf[0] = '\0';
> > > list_for_each_entry(rng, &rng_list, list) {
> > > strncat(buf, rng->name, PAGE_SIZE - ret - 1);
> > > ret += strlen(rng->name);
> > > strncat(buf, " ", PAGE_SIZE - ret - 1);
> > > ret++;
> > > }
> > > + mutex_unlock(&rng_mutex);
> > > strncat(buf, "\n", PAGE_SIZE - ret - 1);
> > > ret++;
> > > - mutex_unlock(&rng_mutex);
> >
> > But this isn't resulting in savings; the majority of the time is being
> > spent in the for loop, and that writes to the buffer.
>
> Right
>
> > BTW I don't expect strcat'ing to the buf in each of these scenarios is
> > a long operation, so this reworking doesn't strike to me as something
> > we should pursue.
> >
> > Amit
>
> --
> Amos.
--
Amos.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2014-09-10 7:32 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
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2014-09-09 15:18 mutex Amos Kong
2014-09-09 15:23 ` RFC virtio-rng: fail to read sysfs of a busy device Amos Kong
2014-09-10 5:52 ` Amit Shah
2014-09-10 6:49 ` Amos Kong
2014-09-10 7:32 ` Amos Kong
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