Robust Futex update

Robust Futex update

[David Singleton]

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Ingo,
    here 's a patch for 2.6.14-rc4-rt1 that fixes two things:

1) Deregister futex returns -EBUSY instead of -EINVAL if a thread tries 
to deregister a pthread_mutex that another thread has locked.

2)  Make the fast path robust.  If a pthread_mutex is only locked in 
user space we need to clear the owner when
the thread dies.  This makes both the 'rt_mutex is locked in the kernel' 
and the 'pthread_mutex is only locked in user space'
paths robust.

David

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Index: linux-2.6.13/kernel/futex.c
===================================================================
--- linux-2.6.13.orig/kernel/futex.c
+++ linux-2.6.13/kernel/futex.c
@@ -1354,18 +1354,25 @@ static void find_owned_futex(struct vm_a
 		if (uaddr == 0)
 			continue;
 
+		up(&mapping->robust_sem);
+		up_read(&current->mm->mmap_sem);
+		value = futex_get_user(uaddr);
+		if (this->futex_mutex.mutex_attr & FUTEX_ATTR_ROBUST)
+			value |= FUTEX_OWNER_DIED;
 		if (rt_mutex_owned_by(&this->futex_mutex, ti)) {
-			up(&mapping->robust_sem);
-			up_read(&current->mm->mmap_sem);
-			if (this->futex_mutex.mutex_attr & FUTEX_ATTR_ROBUST) {
-	 			value = futex_get_user(uaddr);
-				value |= FUTEX_OWNER_DIED;
- 				futex_put_user(value, uaddr);
-			}
+			futex_put_user(value, uaddr);
 			up_futex(&this->futex_mutex);
-			down(&mapping->robust_sem);
-			down_read(&current->mm->mmap_sem);
+		} else if ((value & FUTEX_PID) == current->pid) {
+			/*
+			 * this bit is for the fast path.  If the lock is only
+			 * locked in user space wee need to unlock it
+			 * for the exiting thread.
+			 */
+			value &= ~FUTEX_PID;
+			futex_put_user(value, uaddr);
 		}
+		down(&mapping->robust_sem);
+		down_read(&current->mm->mmap_sem);
 	}
 
 	up(&mapping->robust_sem);
@@ -1524,7 +1531,7 @@ static int futex_deregister(unsigned lon
 			if (rt_mutex_owned_by(&this->futex_mutex, ti)) {
 				up_futex(&this->futex_mutex);
 			} else if (rt_mutex_owner(&this->futex_mutex) != NULL) {
-				ret = -EINVAL;
+				ret = -EBUSY;
 				break;
 			}
 			list_del(&this->list);
Index: linux-2.6.13/include/linux/futex.h
===================================================================
--- linux-2.6.13.orig/include/linux/futex.h
+++ linux-2.6.13/include/linux/futex.h
@@ -1,8 +1,6 @@
 #ifndef _LINUX_FUTEX_H
 #define _LINUX_FUTEX_H
 
-#include <linux/fs.h>
-
 /* Second argument to futex syscall */
 
 
Index: linux-2.6.13/kernel/rt.c
===================================================================
--- linux-2.6.13.orig/kernel/rt.c
+++ linux-2.6.13/kernel/rt.c
@@ -47,6 +47,7 @@
 #include <linux/syscalls.h>
 #include <linux/interrupt.h>
 #include <linux/plist.h>
+#include <linux/fs.h>
 #include <linux/futex.h>
 
 #define CAPTURE_LOCK

Re: Robust Futex update

[Ingo Molnar]

* David Singleton <dsingleton@mvista.com> wrote:

> Ingo,
>    here 's a patch for 2.6.14-rc4-rt1 that fixes two things:
> 
> 1) Deregister futex returns -EBUSY instead of -EINVAL if a thread 
> tries to deregister a pthread_mutex that another thread has locked.
> 
> 2) Make the fast path robust.  If a pthread_mutex is only locked in 
> user space we need to clear the owner when the thread dies.  This 
> makes both the 'rt_mutex is locked in the kernel' and the 
> 'pthread_mutex is only locked in user space' paths robust.

thanks, applied.

	Ingo

Re: Robust Futex update

[Ingo Molnar]

* David Singleton <dsingleton@mvista.com> wrote:

> Index: linux-2.6.13/include/linux/futex.h
> ===================================================================
> --- linux-2.6.13.orig/include/linux/futex.h
> +++ linux-2.6.13/include/linux/futex.h
> @@ -1,8 +1,6 @@
>  #ifndef _LINUX_FUTEX_H
>  #define _LINUX_FUTEX_H
>  
> -#include <linux/fs.h>
> -
>  /* Second argument to futex syscall */
>  

this chunk broke the build, so i added the #include back. Really, the 
robust mutex glibc patches should _NOT_ automatically include the 
kernel's futex.h file. If they do so and rely on an installed 
kernel-headers package then they are broken. Just copy the file into the 
glibc tree and remove the #include line.

	Ingo

Re: Robust Futex update

[Khem Raj]

Hi Ingo

As we pass same flags from glibc to kernel we thought it will be good to 
have defines in one place to avoid going out of sync in future.
glibc already has dependency on kernel headers. So were looking for ways 
to fix futex.h in kernel so that glibc can use it. But if it is not 
feasible then we can define these flags inside glibc headers. Only 
additional burden will be that these headers will be required to be 
synced whenever there is any change pertaining to them(mostly the defines.)

Thanks

Khem

Ingo Molnar wrote:

>* David Singleton <dsingleton@mvista.com> wrote:
>
>  
>
>>Index: linux-2.6.13/include/linux/futex.h
>>===================================================================
>>--- linux-2.6.13.orig/include/linux/futex.h
>>+++ linux-2.6.13/include/linux/futex.h
>>@@ -1,8 +1,6 @@
>> #ifndef _LINUX_FUTEX_H
>> #define _LINUX_FUTEX_H
>> 
>>-#include <linux/fs.h>
>>-
>> /* Second argument to futex syscall */
>> 
>>    
>>
>
>this chunk broke the build, so i added the #include back. Really, the 
>robust mutex glibc patches should _NOT_ automatically include the 
>kernel's futex.h file. If they do so and rely on an installed 
>kernel-headers package then they are broken. Just copy the file into the 
>glibc tree and remove the #include line.
>
>	Ingo
>  
>
>------------------------------------------------------------------------
>
>_______________________________________________
>robustmutexes mailing list
>robustmutexes@lists.osdl.org
>https://lists.osdl.org/mailman/listinfo/robustmutexes
>  
>

-- 
Khem Raj <kraj@mvista.com>
MontaVista Software, Inc.
www.mvista.com