Messages in this thread Patch in this message | | | From | bwatson@kahuna ... | Date | Tue, 12 Feb 2002 14:45 PST | Subject | [PATCH] 2.4.18-pre9, trylock for read/write semaphores |
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Marcelo-
Attached is a patch that adds trylock routines for read/write semaphores. David Howells saw it last August and thought it was ready to be submitted then, but I became distracted and haven't taken the time to submit it until now. My motivation is that Christoph Hellwig says he needs it for JFS.
I hammered on this patch with a test kernel based on 2.4.18-pre7. The test kernel put a read/write semaphore under heavy contention with a mix of down_read(), down_write(), down_read_trylock(), and down_write_trylock() calls. I made sure no process got a lock when it shouldn't, and that no deadlocks occurred. I also made sure it applies cleanly against 2.4.18-pre9. Considering that it does not affect the rest of the kernel (since no one calls it), I think it's safe to include in 2.4.18.
Brian Watson | "Now I don't know, but I been told it's Linux Kernel Developer | hard to run with the weight of gold, Open SSI Clustering Project | Other hand I heard it said, it's Compaq Computer Corp | just as hard with the weight of lead." Los Angeles, CA | -Robert Hunter, 1970
mailto:Brian.J.Watson@compaq.com http://opensource.compaq.com/
diff -Naur linux-2.4.18-pre9/include/asm-i386/rwsem.h trylock-2.4.18-pre9/include/asm-i386/rwsem.h --- linux-2.4.18-pre9/include/asm-i386/rwsem.h Tue Feb 12 12:14:14 2002 +++ trylock-2.4.18-pre9/include/asm-i386/rwsem.h Tue Feb 12 12:14:55 2002 @@ -121,6 +121,24 @@ } /* + * 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) @@ -148,6 +166,19 @@ : "+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; } /* diff -Naur linux-2.4.18-pre9/include/linux/rwsem-spinlock.h trylock-2.4.18-pre9/include/linux/rwsem-spinlock.h --- linux-2.4.18-pre9/include/linux/rwsem-spinlock.h Thu Nov 22 11:46:19 2001 +++ trylock-2.4.18-pre9/include/linux/rwsem-spinlock.h Tue Feb 12 12:14:55 2002 @@ -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)); diff -Naur linux-2.4.18-pre9/include/linux/rwsem.h trylock-2.4.18-pre9/include/linux/rwsem.h --- linux-2.4.18-pre9/include/linux/rwsem.h Thu Nov 22 11:46:19 2001 +++ trylock-2.4.18-pre9/include/linux/rwsem.h Tue Feb 12 12:14:55 2002 @@ -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; } /* diff -Naur linux-2.4.18-pre9/lib/rwsem-spinlock.c trylock-2.4.18-pre9/lib/rwsem-spinlock.c --- linux-2.4.18-pre9/lib/rwsem-spinlock.c Wed Apr 25 13:31:03 2001 +++ trylock-2.4.18-pre9/lib/rwsem-spinlock.c Tue Feb 12 12:14:55 2002 @@ -149,6 +149,28 @@ } /* + * 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 +214,28 @@ 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; } /* - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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