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If we read hwrng by long-running dd process, it takes too much cpu time and almost hold the mutex lock. When we check hwrng attributes from sysfs by cat, it gets stuck in waiting the lock releaseing. The problem can only be reproduced with non-smp guest with slow backend. This patchset resolves the issue by changing rng_dev_read() to always schedule 10 jiffies after release mutex lock, then cat

If we read hwrng by long-running dd process, it takes too much cpu time and almost hold the mutex lock. When we check hwrng attributes from sysfs by cat, it gets stuck in waiting the lock releaseing. The problem can only be reproduced with non-smp guest with slow backend. This patchset resolves the issue by changing rng_dev_read() to always schedule 10 jiffies after release mutex lock, then cat

...s can easy to hold the mutex lock. > > Signed-off-by: Amos Kong <akong at redhat.com> > --- > drivers/char/hw_random/core.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > index 263a370..b5d1b6f 100644 > --- a/drivers/char/hw_random/core.c > +++ b/drivers/char/hw_random/core.c > @@ -195,7 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf, > > mutex_unlock(&rng_mutex); > > - schedule_timeout_interruptible(1); > + schedule...

...s can easy to hold the mutex lock. > > Signed-off-by: Amos Kong <akong at redhat.com> > --- > drivers/char/hw_random/core.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > index 263a370..b5d1b6f 100644 > --- a/drivers/char/hw_random/core.c > +++ b/drivers/char/hw_random/core.c > @@ -195,7 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf, > > mutex_unlock(&rng_mutex); > > - schedule_timeout_interruptible(1); > + schedule...

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

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

...'s more appropriate, then other takes can easy to hold the mutex lock. Signed-off-by: Amos Kong <akong at redhat.com> --- drivers/char/hw_random/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c index 263a370..b5d1b6f 100644 --- a/drivers/char/hw_random/core.c +++ b/drivers/char/hw_random/core.c @@ -195,7 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf, mutex_unlock(&rng_mutex); - schedule_timeout_interruptible(1); + schedule_timeout_interruptible(10); if (sig...

...> > > Signed-off-by: Amos Kong <akong at redhat.com> > > --- > > drivers/char/hw_random/core.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > > index 263a370..b5d1b6f 100644 > > --- a/drivers/char/hw_random/core.c > > +++ b/drivers/char/hw_random/core.c > > @@ -195,7 +195,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf, > > > > mutex_unlock(&rng_mutex); > > > > - schedule_timeout...

...ks can have chance to hold the lock and execute protected code. Signed-off-by: Amos Kong <akong at redhat.com> --- drivers/char/hw_random/core.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c index c591d7e..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)...

...() 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_interrup...

...understand. Grrr.... Rusty. > Signed-off-by: Amos Kong <akong at redhat.com> > --- > drivers/char/hw_random/core.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > index c591d7e..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(...

...understand. Grrr.... Rusty. > Signed-off-by: Amos Kong <akong at redhat.com> > --- > drivers/char/hw_random/core.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c > index c591d7e..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(...