[PATCH RT 0/3] Linux v5.10.90-rt61-rc1

[PATCH RT 0/3] Linux v5.10.90-rt61-rc1

[Luis Claudio R. Goncalves]

Dear RT Folks,

This is the RT stable review cycle of patch 5.10.90-rt61-rc1.

Please scream at me if I messed something up. Please test the patches
too.

The -rc release will be uploaded to kernel.org and will be deleted
when the final release is out. This is just a review release (or
release candidate).

The pre-releases will not be pushed to the git repository, only the
final release is.

If all goes well, this patch will be converted to the next main
release on 2022-01-13.

Known problem:

  There is a printk build warning that would be fixed by commit 43a17111c2553
  ("printk: wake up klogd in vprintk_emit"), already present in v4.19-rt. I am
  trying to decide whether it is justifiable to suggest the backport of this
  commit to 4.14-stable or if I should write a simple rt-only fix. Comments are
  welcome.


To build 5.10.90-rt61-rc1 directly, the following patches should be applied:

  https://www.kernel.org/pub/linux/kernel/v5.x/linux-5.10.tar.xz

  https://www.kernel.org/pub/linux/kernel/v5.x/patch-5.10.90.xz

  https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/patch-5.10.90-rt61-rc1.patch.xz


You can also build from 5.10.90-rt60 by applying the incremental patch:

  https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/incr/patch-5.10.90-rt60-rt61-rc1.patch.xz

Enjoy,

-- Luis



Luis Claudio R. Goncalves (1):
  Linux 5.10.90-rt61-rc1

Sebastian Andrzej Siewior (1):
  stop_machine: Remove this_cpu_ptr() from print_stop_info().

Thomas Gleixner (1):
  eventfd: Make signal recursion protection a task bit

 fs/aio.c                |  2 +-
 fs/eventfd.c            | 12 +++++-------
 include/linux/eventfd.h | 11 +++++------
 include/linux/sched.h   |  4 ++++
 kernel/stop_machine.c   |  6 +++++-
 localversion-rt         |  2 +-
 6 files changed, 21 insertions(+), 16 deletions(-)

-- 
2.33.1

[PATCH RT 1/3] eventfd: Make signal recursion protection a task bit

[Luis Claudio R. Goncalves]

From: Thomas Gleixner <tglx@linutronix.de>

v5.10.90-rt61-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


Upstream commit b542e383d8c005f06a131e2b40d5889b812f19c6

The recursion protection for eventfd_signal() is based on a per CPU
variable and relies on the !RT semantics of spin_lock_irqsave() for
protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither
disables preemption nor interrupts which allows the spin lock held section
to be preempted. If the preempting task invokes eventfd_signal() as well,
then the recursion warning triggers.

Paolo suggested to protect the per CPU variable with a local lock, but
that's heavyweight and actually not necessary. The goal of this protection
is to prevent the task stack from overflowing, which can be achieved with a
per task recursion protection as well.

Replace the per CPU variable with a per task bit similar to other recursion
protection bits like task_struct::in_page_owner. This works on both !RT and
RT kernels and removes as a side effect the extra per CPU storage.

No functional change for !RT kernels.

Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 fs/aio.c                |  2 +-
 fs/eventfd.c            | 12 +++++-------
 include/linux/eventfd.h | 11 +++++------
 include/linux/sched.h   |  4 ++++
 4 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index c72b2c51b446c..f7d47c9ff6deb 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1761,7 +1761,7 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 		list_del_init(&req->wait.entry);
 		list_del(&iocb->ki_list);
 		iocb->ki_res.res = mangle_poll(mask);
-		if (iocb->ki_eventfd && eventfd_signal_count()) {
+		if (iocb->ki_eventfd && eventfd_signal_allowed()) {
 			iocb = NULL;
 			INIT_WORK(&req->work, aio_poll_put_work);
 			schedule_work(&req->work);
diff --git a/fs/eventfd.c b/fs/eventfd.c
index df466ef81dddf..9035ca60bfcf3 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -25,8 +25,6 @@
 #include <linux/idr.h>
 #include <linux/uio.h>
 
-DEFINE_PER_CPU(int, eventfd_wake_count);
-
 static DEFINE_IDA(eventfd_ida);
 
 struct eventfd_ctx {
@@ -67,21 +65,21 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
 	 * Deadlock or stack overflow issues can happen if we recurse here
 	 * through waitqueue wakeup handlers. If the caller users potentially
 	 * nested waitqueues with custom wakeup handlers, then it should
-	 * check eventfd_signal_count() before calling this function. If
-	 * it returns true, the eventfd_signal() call should be deferred to a
+	 * check eventfd_signal_allowed() before calling this function. If
+	 * it returns false, the eventfd_signal() call should be deferred to a
 	 * safe context.
 	 */
-	if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
+	if (WARN_ON_ONCE(current->in_eventfd_signal))
 		return 0;
 
 	spin_lock_irqsave(&ctx->wqh.lock, flags);
-	this_cpu_inc(eventfd_wake_count);
+	current->in_eventfd_signal = 1;
 	if (ULLONG_MAX - ctx->count < n)
 		n = ULLONG_MAX - ctx->count;
 	ctx->count += n;
 	if (waitqueue_active(&ctx->wqh))
 		wake_up_locked_poll(&ctx->wqh, EPOLLIN);
-	this_cpu_dec(eventfd_wake_count);
+	current->in_eventfd_signal = 0;
 	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 
 	return n;
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index dc4fd8a6644dd..836b4c021a0a4 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -14,6 +14,7 @@
 #include <linux/err.h>
 #include <linux/percpu-defs.h>
 #include <linux/percpu.h>
+#include <linux/sched.h>
 
 /*
  * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
@@ -42,11 +43,9 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n);
 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
 				  __u64 *cnt);
 
-DECLARE_PER_CPU(int, eventfd_wake_count);
-
-static inline bool eventfd_signal_count(void)
+static inline bool eventfd_signal_allowed(void)
 {
-	return this_cpu_read(eventfd_wake_count);
+	return !current->in_eventfd_signal;
 }
 
 #else /* CONFIG_EVENTFD */
@@ -77,9 +76,9 @@ static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
 	return -ENOSYS;
 }
 
-static inline bool eventfd_signal_count(void)
+static inline bool eventfd_signal_allowed(void)
 {
-	return false;
+	return true;
 }
 
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 409a24036952c..29e6ff1af1df9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -852,6 +852,10 @@ struct task_struct {
 	/* Stalled due to lack of memory */
 	unsigned			in_memstall:1;
 #endif
+#ifdef CONFIG_EVENTFD
+	/* Recursion prevention for eventfd_signal() */
+	unsigned			in_eventfd_signal:1;
+#endif
 
 	unsigned long			atomic_flags; /* Flags requiring atomic access. */
 
-- 
2.33.1

[PATCH RT 2/3] stop_machine: Remove this_cpu_ptr() from print_stop_info().

[Luis Claudio R. Goncalves]

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

v5.10.90-rt61-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


This aligns the patch ("stop_machine: Add function and caller debug
info) with commit
  a8b62fd085050 ("stop_machine: Add function and caller debug info")

that was merged upstream and is slightly different.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/stop_machine.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index dbf585cf4b9f8..971d8acceaecb 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -51,7 +51,11 @@ static bool stop_machine_initialized = false;
 
 void print_stop_info(const char *log_lvl, struct task_struct *task)
 {
-	struct cpu_stopper *stopper = this_cpu_ptr(&cpu_stopper);
+	/*
+	 * If @task is a stopper task, it cannot migrate and task_cpu() is
+	 * stable.
+	 */
+	struct cpu_stopper *stopper = per_cpu_ptr(&cpu_stopper, task_cpu(task));
 
 	if (task != stopper->thread)
 		return;
-- 
2.33.1

[PATCH RT 3/3] Linux 5.10.90-rt61-rc1

[Luis Claudio R. Goncalves]

v5.10.90-rt61-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
---
 localversion-rt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/localversion-rt b/localversion-rt
index 66fa05e70f292..c040746653492 100644
--- a/localversion-rt
+++ b/localversion-rt
@@ -1 +1 @@
--rt60
+-rt61-rc1
-- 
2.33.1

Re: [PATCH RT 0/3] Linux v5.10.90-rt61-rc1

[Luis Goncalves]

On Thu, Jan 6, 2022 at 11:15 PM Luis Claudio R. Goncalves
<lgoncalv@redhat.com> wrote:
>
> Dear RT Folks,
>
> This is the RT stable review cycle of patch 5.10.90-rt61-rc1.
>
> Please scream at me if I messed something up. Please test the patches
> too.
>
> The -rc release will be uploaded to kernel.org and will be deleted
> when the final release is out. This is just a review release (or
> release candidate).
>
> The pre-releases will not be pushed to the git repository, only the
> final release is.
>
> If all goes well, this patch will be converted to the next main
> release on 2022-01-13.
>
> Known problem:
>
>   There is a printk build warning that would be fixed by commit 43a17111c2553
>   ("printk: wake up klogd in vprintk_emit"), already present in v4.19-rt. I am
>   trying to decide whether it is justifiable to suggest the backport of this
>   commit to 4.14-stable or if I should write a simple rt-only fix. Comments are
>   welcome.

Please disregard the note above. That was an error in my template message.
There are no known errors in this -rc1 release.

Sorry for any inconvenience.

> To build 5.10.90-rt61-rc1 directly, the following patches should be applied:
>
>   https://www.kernel.org/pub/linux/kernel/v5.x/linux-5.10.tar.xz
>
>   https://www.kernel.org/pub/linux/kernel/v5.x/patch-5.10.90.xz
>
>   https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/patch-5.10.90-rt61-rc1.patch.xz
>
>
> You can also build from 5.10.90-rt60 by applying the incremental patch:
>
>   https://www.kernel.org/pub/linux/kernel/projects/rt/5.10/incr/patch-5.10.90-rt60-rt61-rc1.patch.xz
>
> Enjoy,
>
> -- Luis
>
>
>
> Luis Claudio R. Goncalves (1):
>   Linux 5.10.90-rt61-rc1
>
> Sebastian Andrzej Siewior (1):
>   stop_machine: Remove this_cpu_ptr() from print_stop_info().
>
> Thomas Gleixner (1):
>   eventfd: Make signal recursion protection a task bit
>
>  fs/aio.c                |  2 +-
>  fs/eventfd.c            | 12 +++++-------
>  include/linux/eventfd.h | 11 +++++------
>  include/linux/sched.h   |  4 ++++
>  kernel/stop_machine.c   |  6 +++++-
>  localversion-rt         |  2 +-
>  6 files changed, 21 insertions(+), 16 deletions(-)
>
> --
> 2.33.1
>

Re: [PATCH RT 1/3] eventfd: Make signal recursion protection a task bit

[Daniel Vacek]

Hi,

OT: Btw for some reason you're sending a copy to
"stable-rt@vger.kernel.org"@redhat.com.

On Fri, Jan 7, 2022 at 8:04 PM Luis Claudio R. Goncalves
<lgoncalv@redhat.com> wrote:
>
> From: Thomas Gleixner <tglx@linutronix.de>
>
> v5.10.90-rt61-rc1 stable review patch.
> If anyone has any objections, please let me know.
>
> -----------
>
>
> Upstream commit b542e383d8c005f06a131e2b40d5889b812f19c6
>
> The recursion protection for eventfd_signal() is based on a per CPU
> variable and relies on the !RT semantics of spin_lock_irqsave() for
> protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither
> disables preemption nor interrupts which allows the spin lock held section
> to be preempted. If the preempting task invokes eventfd_signal() as well,
> then the recursion warning triggers.
>
> Paolo suggested to protect the per CPU variable with a local lock, but
> that's heavyweight and actually not necessary. The goal of this protection
> is to prevent the task stack from overflowing, which can be achieved with a
> per task recursion protection as well.
>
> Replace the per CPU variable with a per task bit similar to other recursion
> protection bits like task_struct::in_page_owner. This works on both !RT and
> RT kernels and removes as a side effect the extra per CPU storage.
>
> No functional change for !RT kernels.
>
> Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> Acked-by: Jason Wang <jasowang@redhat.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  fs/aio.c                |  2 +-
>  fs/eventfd.c            | 12 +++++-------
>  include/linux/eventfd.h | 11 +++++------
>  include/linux/sched.h   |  4 ++++
>  4 files changed, 15 insertions(+), 14 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index c72b2c51b446c..f7d47c9ff6deb 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1761,7 +1761,7 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
>                 list_del_init(&req->wait.entry);
>                 list_del(&iocb->ki_list);
>                 iocb->ki_res.res = mangle_poll(mask);
> -               if (iocb->ki_eventfd && eventfd_signal_count()) {
> +               if (iocb->ki_eventfd && eventfd_signal_allowed()) {

IIUC, the logic changed and this should read
!eventfd_signal_allowed(). Or am I missing something here?

Actually checking other stable branches and upstream, this looks to be
a typo only in v5.10.90-rt61-rc1

--nX




>                         iocb = NULL;
>                         INIT_WORK(&req->work, aio_poll_put_work);
>                         schedule_work(&req->work);
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index df466ef81dddf..9035ca60bfcf3 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -25,8 +25,6 @@
>  #include <linux/idr.h>
>  #include <linux/uio.h>
>
> -DEFINE_PER_CPU(int, eventfd_wake_count);
> -
>  static DEFINE_IDA(eventfd_ida);
>
>  struct eventfd_ctx {
> @@ -67,21 +65,21 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
>          * Deadlock or stack overflow issues can happen if we recurse here
>          * through waitqueue wakeup handlers. If the caller users potentially
>          * nested waitqueues with custom wakeup handlers, then it should
> -        * check eventfd_signal_count() before calling this function. If
> -        * it returns true, the eventfd_signal() call should be deferred to a
> +        * check eventfd_signal_allowed() before calling this function. If
> +        * it returns false, the eventfd_signal() call should be deferred to a
>          * safe context.
>          */
> -       if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
> +       if (WARN_ON_ONCE(current->in_eventfd_signal))
>                 return 0;
>
>         spin_lock_irqsave(&ctx->wqh.lock, flags);
> -       this_cpu_inc(eventfd_wake_count);
> +       current->in_eventfd_signal = 1;
>         if (ULLONG_MAX - ctx->count < n)
>                 n = ULLONG_MAX - ctx->count;
>         ctx->count += n;
>         if (waitqueue_active(&ctx->wqh))
>                 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
> -       this_cpu_dec(eventfd_wake_count);
> +       current->in_eventfd_signal = 0;
>         spin_unlock_irqrestore(&ctx->wqh.lock, flags);
>
>         return n;
> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> index dc4fd8a6644dd..836b4c021a0a4 100644
> --- a/include/linux/eventfd.h
> +++ b/include/linux/eventfd.h
> @@ -14,6 +14,7 @@
>  #include <linux/err.h>
>  #include <linux/percpu-defs.h>
>  #include <linux/percpu.h>
> +#include <linux/sched.h>
>
>  /*
>   * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
> @@ -42,11 +43,9 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n);
>  int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
>                                   __u64 *cnt);
>
> -DECLARE_PER_CPU(int, eventfd_wake_count);
> -
> -static inline bool eventfd_signal_count(void)
> +static inline bool eventfd_signal_allowed(void)
>  {
> -       return this_cpu_read(eventfd_wake_count);
> +       return !current->in_eventfd_signal;
>  }
>
>  #else /* CONFIG_EVENTFD */
> @@ -77,9 +76,9 @@ static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
>         return -ENOSYS;
>  }
>
> -static inline bool eventfd_signal_count(void)
> +static inline bool eventfd_signal_allowed(void)
>  {
> -       return false;
> +       return true;
>  }
>
>  #endif
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 409a24036952c..29e6ff1af1df9 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -852,6 +852,10 @@ struct task_struct {
>         /* Stalled due to lack of memory */
>         unsigned                        in_memstall:1;
>  #endif
> +#ifdef CONFIG_EVENTFD
> +       /* Recursion prevention for eventfd_signal() */
> +       unsigned                        in_eventfd_signal:1;
> +#endif
>
>         unsigned long                   atomic_flags; /* Flags requiring atomic access. */
>
> --
> 2.33.1
>

Re: [PATCH RT 1/3] eventfd: Make signal recursion protection a task bit

[Daniel Vacek]

Actually I see it was fixed later with
4b3749865374899e115aa8c48681709b086fe6d3 so I guess it should be
included as well.

--nX

On Thu, Feb 3, 2022 at 9:59 AM Daniel Vacek <neelx.g@gmail.com> wrote:
>
> Hi,
>
> OT: Btw for some reason you're sending a copy to
> "stable-rt@vger.kernel.org"@redhat.com.
>
> On Fri, Jan 7, 2022 at 8:04 PM Luis Claudio R. Goncalves
> <lgoncalv@redhat.com> wrote:
> >
> > From: Thomas Gleixner <tglx@linutronix.de>
> >
> > v5.10.90-rt61-rc1 stable review patch.
> > If anyone has any objections, please let me know.
> >
> > -----------
> >
> >
> > Upstream commit b542e383d8c005f06a131e2b40d5889b812f19c6
> >
> > The recursion protection for eventfd_signal() is based on a per CPU
> > variable and relies on the !RT semantics of spin_lock_irqsave() for
> > protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither
> > disables preemption nor interrupts which allows the spin lock held section
> > to be preempted. If the preempting task invokes eventfd_signal() as well,
> > then the recursion warning triggers.
> >
> > Paolo suggested to protect the per CPU variable with a local lock, but
> > that's heavyweight and actually not necessary. The goal of this protection
> > is to prevent the task stack from overflowing, which can be achieved with a
> > per task recursion protection as well.
> >
> > Replace the per CPU variable with a per task bit similar to other recursion
> > protection bits like task_struct::in_page_owner. This works on both !RT and
> > RT kernels and removes as a side effect the extra per CPU storage.
> >
> > No functional change for !RT kernels.
> >
> > Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> > Acked-by: Jason Wang <jasowang@redhat.com>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Link: https://lore.kernel.org/r/87wnp9idso.ffs@tglx
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  fs/aio.c                |  2 +-
> >  fs/eventfd.c            | 12 +++++-------
> >  include/linux/eventfd.h | 11 +++++------
> >  include/linux/sched.h   |  4 ++++
> >  4 files changed, 15 insertions(+), 14 deletions(-)
> >
> > diff --git a/fs/aio.c b/fs/aio.c
> > index c72b2c51b446c..f7d47c9ff6deb 100644
> > --- a/fs/aio.c
> > +++ b/fs/aio.c
> > @@ -1761,7 +1761,7 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
> >                 list_del_init(&req->wait.entry);
> >                 list_del(&iocb->ki_list);
> >                 iocb->ki_res.res = mangle_poll(mask);
> > -               if (iocb->ki_eventfd && eventfd_signal_count()) {
> > +               if (iocb->ki_eventfd && eventfd_signal_allowed()) {
>
> IIUC, the logic changed and this should read
> !eventfd_signal_allowed(). Or am I missing something here?
>
> Actually checking other stable branches and upstream, this looks to be
> a typo only in v5.10.90-rt61-rc1
>
> --nX
>
>
>
>
> >                         iocb = NULL;
> >                         INIT_WORK(&req->work, aio_poll_put_work);
> >                         schedule_work(&req->work);
> > diff --git a/fs/eventfd.c b/fs/eventfd.c
> > index df466ef81dddf..9035ca60bfcf3 100644
> > --- a/fs/eventfd.c
> > +++ b/fs/eventfd.c
> > @@ -25,8 +25,6 @@
> >  #include <linux/idr.h>
> >  #include <linux/uio.h>
> >
> > -DEFINE_PER_CPU(int, eventfd_wake_count);
> > -
> >  static DEFINE_IDA(eventfd_ida);
> >
> >  struct eventfd_ctx {
> > @@ -67,21 +65,21 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
> >          * Deadlock or stack overflow issues can happen if we recurse here
> >          * through waitqueue wakeup handlers. If the caller users potentially
> >          * nested waitqueues with custom wakeup handlers, then it should
> > -        * check eventfd_signal_count() before calling this function. If
> > -        * it returns true, the eventfd_signal() call should be deferred to a
> > +        * check eventfd_signal_allowed() before calling this function. If
> > +        * it returns false, the eventfd_signal() call should be deferred to a
> >          * safe context.
> >          */
> > -       if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
> > +       if (WARN_ON_ONCE(current->in_eventfd_signal))
> >                 return 0;
> >
> >         spin_lock_irqsave(&ctx->wqh.lock, flags);
> > -       this_cpu_inc(eventfd_wake_count);
> > +       current->in_eventfd_signal = 1;
> >         if (ULLONG_MAX - ctx->count < n)
> >                 n = ULLONG_MAX - ctx->count;
> >         ctx->count += n;
> >         if (waitqueue_active(&ctx->wqh))
> >                 wake_up_locked_poll(&ctx->wqh, EPOLLIN);
> > -       this_cpu_dec(eventfd_wake_count);
> > +       current->in_eventfd_signal = 0;
> >         spin_unlock_irqrestore(&ctx->wqh.lock, flags);
> >
> >         return n;
> > diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> > index dc4fd8a6644dd..836b4c021a0a4 100644
> > --- a/include/linux/eventfd.h
> > +++ b/include/linux/eventfd.h
> > @@ -14,6 +14,7 @@
> >  #include <linux/err.h>
> >  #include <linux/percpu-defs.h>
> >  #include <linux/percpu.h>
> > +#include <linux/sched.h>
> >
> >  /*
> >   * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
> > @@ -42,11 +43,9 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n);
> >  int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_entry_t *wait,
> >                                   __u64 *cnt);
> >
> > -DECLARE_PER_CPU(int, eventfd_wake_count);
> > -
> > -static inline bool eventfd_signal_count(void)
> > +static inline bool eventfd_signal_allowed(void)
> >  {
> > -       return this_cpu_read(eventfd_wake_count);
> > +       return !current->in_eventfd_signal;
> >  }
> >
> >  #else /* CONFIG_EVENTFD */
> > @@ -77,9 +76,9 @@ static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
> >         return -ENOSYS;
> >  }
> >
> > -static inline bool eventfd_signal_count(void)
> > +static inline bool eventfd_signal_allowed(void)
> >  {
> > -       return false;
> > +       return true;
> >  }
> >
> >  #endif
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 409a24036952c..29e6ff1af1df9 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -852,6 +852,10 @@ struct task_struct {
> >         /* Stalled due to lack of memory */
> >         unsigned                        in_memstall:1;
> >  #endif
> > +#ifdef CONFIG_EVENTFD
> > +       /* Recursion prevention for eventfd_signal() */
> > +       unsigned                        in_eventfd_signal:1;
> > +#endif
> >
> >         unsigned long                   atomic_flags; /* Flags requiring atomic access. */
> >
> > --
> > 2.33.1
> >

Re: [PATCH RT 1/3] eventfd: Make signal recursion protection a task bit

[Luis Goncalves]

On Thu, Feb 3, 2022 at 6:17 AM Daniel Vacek <neelx.g@gmail.com> wrote:
>
> Actually I see it was fixed later with
> 4b3749865374899e115aa8c48681709b086fe6d3 so I guess it should be
> included as well.

Thank you for the feedback!
I will proceed with the changes and testing to release the kernel
probably on Monday.

Also, thank you for the note about the wrong email address!

Best regards,
Luis