Re: [PATCH] 2.4.7 read/write semaphore trylock routines

Brian J. Watson (Brian.J.Watson@compaq.com)
Fri, 27 Jul 2001 15:33:29 -0700

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"Brian J. Watson" wrote:
>
> David Howells wrote:
> >
> > > Is this patch ready to go into the kernel? Is there anything else I
> > > should do to get it ready? I noticed it didn't make it into
> > > 2.4.7-pre7.
> >
> > It looks reasonable, though I haven't tried it myself (I presume you
> > have
> > though). You might want to try sending it direct to Linus.
> >
> > David
>
> Good thing I hammered on the trylock routines. The i386 version was fine, but
> in the generic version I wasn't dropping the spinlock under contention. My
> test machine was kind enough to deadlock to make my mistake painfully obvious.
>
> The trylock patch with test code can be found below. Aside from fixing the
> deadlock above, I made sure the lock was behaving properly. I'll send the
> patch sans test code to Linus.
>
> --
> Brian Watson | "The common people of England... so
> Linux Kernel Developer | jealous of their liberty, but like the
> Open SSI Clustering Project | common people of most other countries
> Compaq Computer Corp | never rightly considering wherein it
> Los Angeles, CA | consists..."
> | -Adam Smith, Wealth of Nations, 1776
>
> mailto:Brian.J.Watson@compaq.com
> http://opensource.compaq.com/
>
> == cut ==
> --- ../generic-hash/linux-2.4.7/include/asm-i386/rwsem.h Fri Apr 27 15:48:24 2001
> +++ linux/include/asm-i386/rwsem.h Thu Jul 26 18:59:11 2001
> @@ -117,6 +117,24 @@
> }
>
> /*
> + * trylock for reading -- returns 1 if successful, 0 if contention
> + */
> +static inline int __down_read_trylock(struct rw_semaphore *sem)
> +{
> + signed long old, new;
> +
> +repeat:
> + old = (volatile signed long)sem->count;
> + if (old < RWSEM_UNLOCKED_VALUE)
> + return 0;
> + new = old + RWSEM_ACTIVE_READ_BIAS;
> + if (cmpxchg(&sem->count, old, new) == old)
> + return 1;
> + else
> + goto repeat;
> +}
> +
> +/*
> * lock for writing
> */
> static inline void __down_write(struct rw_semaphore *sem)
> @@ -141,6 +159,19 @@
> : "+d"(tmp), "+m"(sem->count)
> : "a"(sem)
> : "memory", "cc");
> +}
> +
> +/*
> + * trylock for writing -- returns 1 if successful, 0 if contention
> + */
> +static inline int __down_write_trylock(struct rw_semaphore *sem)
> +{
> + signed long ret = cmpxchg(&sem->count,
> + RWSEM_UNLOCKED_VALUE,
> + RWSEM_ACTIVE_WRITE_BIAS);
> + if (ret == RWSEM_UNLOCKED_VALUE)
> + return 1;
> + return 0;
> }
>
> /*
> --- ../generic-hash/linux-2.4.7/include/linux/rwsem-spinlock.h Thu Jul 26 14:35:50 2001
> +++ linux/include/linux/rwsem-spinlock.h Thu Jul 26 18:59:10 2001
> @@ -54,7 +54,9 @@
>
> extern void FASTCALL(init_rwsem(struct rw_semaphore *sem));
> extern void FASTCALL(__down_read(struct rw_semaphore *sem));
> +extern int FASTCALL(__down_read_trylock(struct rw_semaphore *sem));
> extern void FASTCALL(__down_write(struct rw_semaphore *sem));
> +extern int FASTCALL(__down_write_trylock(struct rw_semaphore *sem));
> extern void FASTCALL(__up_read(struct rw_semaphore *sem));
> extern void FASTCALL(__up_write(struct rw_semaphore *sem));
>
> --- ../generic-hash/linux-2.4.7/include/linux/rwsem.h Thu Jul 26 14:35:50 2001
> +++ linux/include/linux/rwsem.h Fri Jul 27 15:08:44 2001
> @@ -46,6 +46,18 @@
> }
>
> /*
> + * trylock for reading -- returns 1 if successful, 0 if contention
> + */
> +static inline int down_read_trylock(struct rw_semaphore *sem)
> +{
> + int ret;
> + rwsemtrace(sem,"Entering down_read_trylock");
> + ret = __down_read_trylock(sem);
> + rwsemtrace(sem,"Leaving down_read_trylock");
> + return ret;
> +}
> +
> +/*
> * lock for writing
> */
> static inline void down_write(struct rw_semaphore *sem)
> @@ -53,6 +65,18 @@
> rwsemtrace(sem,"Entering down_write");
> __down_write(sem);
> rwsemtrace(sem,"Leaving down_write");
> +}
> +
> +/*
> + * trylock for writing -- returns 1 if successful, 0 if contention
> + */
> +static inline int down_write_trylock(struct rw_semaphore *sem)
> +{
> + int ret;
> + rwsemtrace(sem,"Entering down_write_trylock");
> + ret = __down_write_trylock(sem);
> + rwsemtrace(sem,"Leaving down_write_trylock");
> + return ret;
> }
>
> /*
> --- ../generic-hash/linux-2.4.7/include/rwsem_test.h Fri Jul 27 15:07:17 2001
> +++ linux/include/rwsem_test.h Thu Jul 26 17:26:30 2001
> @@ -0,0 +1,82 @@
> +#include <linux/sched.h>
> +#include <linux/types.h>
> +#include <linux/random.h>
> +#include <linux/rwsem.h>
> +
> +DECLARE_RWSEM(rst_lock);
> +
> +int
> +rst_daemon(void *ignore)
> +{
> + while (1) {
> + unsigned char sleep, style, ctr = 0;
> +
> + get_random_bytes(&sleep, 1);
> + __set_current_state(TASK_UNINTERRUPTIBLE);
> + schedule_timeout((signed long)sleep);
> +
> + for (; ctr < 10; ++ctr) {
> + get_random_bytes(&style, 1);
> +
> + if (style > 191) {
> + down_write(&rst_lock);
> + printk("[rst %d] down_write\n", current->pid);
> + } else if (style > 127) {
> + if (!down_write_trylock(&rst_lock)) {
> + printk("[rst %d] down_write_trylock "
> + "failed\n",
> + current->pid);
> + break;
> + }
> + printk("[rst %d] down_write_trylock "
> + "succeeded\n", current->pid);
> + } else if (style > 63) {
> + down_read(&rst_lock);
> + printk("[rst %d] down_read\n", current->pid);
> + } else {
> + if (!down_read_trylock(&rst_lock)) {
> + printk("[rst %d] down_read_trylock "
> + "failed\n",
> + current->pid);
> + break;
> + }
> + printk("[rst %d] down_read_trylock "
> + "succeeded\n", current->pid);
> + }
> +
> + if (style > 127)
> + printk("[rst %d] writing... [1]\n",
> + current->pid);
> + else
> + printk("[rst %d] reading... [1]\n",
> + current->pid);
> +
> + __set_current_state(TASK_UNINTERRUPTIBLE);
> + schedule_timeout(5);
> +
> + if (style > 127)
> + printk("[rst %d] writing... [2]\n",
> + current->pid);
> + else
> + printk("[rst %d] reading... [2]\n",
> + current->pid);
> +
> + if (style > 127) {
> + printk("[rst %d] up_write\n", current->pid);
> + up_write(&rst_lock);
> + } else {
> + printk("[rst %d] up_read\n", current->pid);
> + up_read(&rst_lock);
> + }
> + }
> + }
> + return 0;
> +}
> +
> +void
> +rst_init(void)
> +{
> + int ctr = 0;
> + for (; ctr < 10; ++ctr)
> + kernel_thread(rst_daemon, NULL, 0);
> +}
> --- ../generic-hash/linux-2.4.7/init/main.c Thu Jul 26 18:33:53 2001
> +++ linux/init/main.c Fri Jul 27 14:54:18 2001
> @@ -11,6 +11,8 @@
>
> #define __KERNEL_SYSCALLS__
>
> +#include <rwsem_test.h>
> +
> #include <linux/config.h>
> #include <linux/proc_fs.h>
> #include <linux/devfs_fs_kernel.h>
> @@ -799,6 +801,8 @@
> * The Bourne shell can be used instead of init if we are
> * trying to recover a really broken machine.
> */
> +
> + rst_init();
>
> if (execute_command)
> execve(execute_command,argv_init,envp_init);
> --- ../generic-hash/linux-2.4.7/lib/rwsem-spinlock.c Wed Apr 25 13:31:03 2001
> +++ linux/lib/rwsem-spinlock.c Fri Jul 27 15:08:44 2001
> @@ -149,6 +149,26 @@
> }
>
> /*
> + * trylock for reading -- returns 1 if successful, 0 if contention
> + */
> +int __down_read_trylock(struct rw_semaphore *sem)
> +{
> + int ret = 0;
> + rwsemtrace(sem,"Entering __down_read_trylock");
> +
> + spin_lock(&sem->wait_lock);
> + if (sem->activity>=0 && list_empty(&sem->wait_list)) {
> + /* granted */
> + sem->activity++;
> + ret = 1;
> + }
> + spin_unlock(&sem->wait_lock);
> +
> + rwsemtrace(sem,"Leaving __down_read_trylock");
> + return ret;
> +}
> +
> +/*
> * get a write lock on the semaphore
> * - note that we increment the waiting count anyway to indicate an exclusive lock
> */
> @@ -192,6 +212,26 @@
>
> out:
> rwsemtrace(sem,"Leaving __down_write");
> +}
> +
> +/*
> + * trylock for writing -- returns 1 if successful, 0 if contention
> + */
> +int __down_write_trylock(struct rw_semaphore *sem)
> +{
> + int ret = 0;
> + rwsemtrace(sem,"Entering __down_write_trylock");
> +
> + spin_lock(&sem->wait_lock);
> + if (sem->activity==0 && list_empty(&sem->wait_list)) {
> + /* granted */
> + sem->activity = -1;
> + ret = 1;
> + }
> + spin_unlock(&sem->wait_lock);
> +
> + rwsemtrace(sem,"Leaving __down_write_trylock");
> + return ret;
> }
>
> /*

-- Brian Watson | "The common people of England... so Linux Kernel Developer | jealous of their liberty, but like the Open SSI Clustering Project | common people of most other countries Compaq Computer Corp | never rightly considering wherein it Los Angeles, CA | consists..." | -Adam Smith, Wealth of Nations, 1776

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Next message: Brian J. Watson: "Re: [PATCH] 2.4.7 read/write semaphore trylock routines"
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